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A birth defect is an abnormality of structure or function in a person, which is present from birth. The birth defect may be clinically obvious at birth, or may only be diagnosed sometime later in life. For example, a neural tube defect is a structural defect which is obvious at birth while haemophilia, which is also present at birth, is a functional defect that may only become obvious and be diagnosed when the child is older. Birth defects often present as an abnormal appearance or failure to grow and develop normally.
A birth defect is a structural or functional abnormality which is present from birth.
Congenital disorder is another term that has the same meaning and definition as birth defect. Congenital means ‘present at birth’.
Malformations are the commonest form of birth defect. Congenital malformations develop during the first trimester and are caused by failure of the embryo to develop normally. This results in a birth defect of one or more organs (e.g. heart, eye, brain).
Congenital malformations occur early in pregnancy when the embryo is still forming.
No. Birth defects may be mild or serious. A mild defect causes no disability. However, a person with a serious birth defect may die soon after birth, or survive with a disability due to the direct effect of the birth defect (e.g. neural tube defect) or due to a secondary effect (e.g. joint damage resulting from bleeding in haemophilia). Some serious birth defects can be treated and this may be life-saving or prevent or reduce serious disability.
Birth defects can cause a wide range of disability, e.g. physical disability, intellectual disability, blindness, deafness and epilepsy.
Serious birth defects can cause death or disability.
The frequency of birth defects (i.e. how common are individuals with a birth defect) is expressed as population prevalence and birth prevalence:
At birth two to three percent of live newborn infants can be recognised as having a birth defect, i.e. the recognisable birth prevalence for all defects is 20–30/1000 live births in the first month of life.
However, not all birth defects are diagnosed at or around birth, and by five years of age between four and eight percent of children in different countries are considered to have suffered the effects of a serious birth defect, i.e. a birth prevalence of 40–80/1000 live births.
The birth prevalence of serious birth defects varies from 40/1000 live births in industrialised countries to as high as 80/1000 in some developing countries.
The birth prevalence of serious birth defects is lower in industrial countries than in developing countries.
Therefore both the birth prevalence and population prevalence of birth defects in developing countries may seem to be much lower than it really is.
In South Africa about 1 million infants are born annually. Based on the available evidence, about 72 000 infants are born each year with a severe birth defect. Of these infants, about 25% will die in the first five years of life.
It is estimated that 9 million children are born in the world each year with a serious birth defect. Of these children, at least 8.4 million (93%) are born in developing countries. A minimum of 3.3 million children with a serious birth defect are estimated to die annually.
Birth defects are an important cause of infant and childhood death.
Birth defects are caused by:
These are also known as genetic causes of birth defects.
These are non-genetic causes of birth defects. Note that all birth defects are not due to genetic causes.
The cause of about 50% of birth defects is not yet known.
Chromosomes are packages of DNA (deoxyribonucleic acid), the genetic material found in all cells. A person’s genetic plan of all their inherited characteristics is stored in their chromosomes.
Human cells have 46 chromosomes that are contained in the nucleus of the cell. The chromosomes are paired (23 pairs), with 22 pairs called autosomes and one pair of sex chromosomes. Each pair of autosomes looks the same. The pair of sex chromosomes do not look the same because the X chromosome is longer than the Y chromosome.
Females have two X chromosomes (i.e. XX) while males have one X and one Y chromosome (i.e. XY). Like the 22 autosomes, the pair of X chromosomes in females look alike.
A picture of the 46 chromosomes is called a karyotype. The normal female karyotype can be written as 46,XX and the normal male karyotype as 46,XY. Each pair of autosomes is given a number (1 to 22).
Humans have 46 chromosomes in each cell, 22 pairs of autosomes and one pair of sex chromosomes.
Figure 1-1: Normal karyotype of a male (46,XY) with 23 pairs of chromosomes (22 pairs of matching autosomes and one pair of unlike sex chromosomes, X and Y)
One chromosome of each pair of chromosomes is inherited from the mother and the other chromosome from the father. Therefore, both the mother and father give one chromosome to each pair of chromosomes found in the child. Half of the inheritance plan of each individual is inherited from the mother and the other half from the father. This is called sexual reproduction. An infant’s genetic plan is, therefore, inherited from both parents. This is why the inherited characteristics of the parents are shared in the child, and the child has features of both parents.
When the ova (female eggs) are produced in the mother’s ovaries, and the sperms (male eggs) in the father’s testicles, the 46 chromosomes in the parent’s stem cells divide with only one copy of each chromosome pair still remaining in each ovum or sperm. The ova and sperms (also called gametes or sex cells), therefore, only have 23 chromosomes each.
With fertilisation, a sperm and an ovum unite and combine their chromosomes to form the zygote (the first cell which will eventually develop into the fetus). The zygote therefore has 46 chromosomes, half (23) from the mother and half (23) from the father. The zygote divides, multiplies and grows to become an embryo (with cells developing into different organs). The embryo develops into the fetus (with formed organs). After delivery the fetus is called the newborn infant.
Each parent gives 23 chromosomes which combine at fertilisation to give a total of 46 chromosomes in the zygote.
Cell division in which the chromosome number stays the same can also occur (asexual reproduction) and this is called mitosis. This is the type of cell reproduction that occurs to make more cells so that the zygote can multiply and develop into an embryo and fetus, and the body can grow or replace cells that die off during life.
Figure 1-2: The normal chromosome contribution of each parent
The process of reproduction (when the ova and sperms are made, fertilised and divide after conception) is not always perfect. Abnormalities can occur in the chromosomes and they may result in a child with a birth defect. These chromosome abnormalities are mostly sporadic, i.e. due to chance.
Chromosome abnormalities include:
If a whole chromosome or part of a chromosome is gained or lost in the process of reproduction, then the zygote that results will be abnormal as its genetic plan will have more or less genetic information than it should have. The abnormal embryo may abort spontaneously or result in an infant with a birth defect. Chromosomal disorders usually present with multiple abnormalities, including an abnormal appearance (dysmorphic features), developmental and growth delay and malformations. As most chromosomal abnormalities are not inherited, the risk of more than one child being affected (recurrence) is low.
The risk of the same chromosome disorder occurring more than once in a family is low.
This occurs during the formation of the gametes (ova or sperms) when a pair of the parent’s chromosome does not split normally. Instead of one chromosome of a pair going to each gamete, one gamete gets both the paired chromosomes, and therefore has 24 chromosomes, while the other gamete does not get a copy of that chromosome and, therefore, only has 22 chromosomes. This abnormal process of cell division, which results in two abnormal gametes, is known as non-disjunction.
When either of these two abnormal gametes fertilise with a normal gamete (containing 23 chromosomes) the resulting zygote will have either of the following:
From an abnormal zygote with 47 chromosomes (trisomy) the fetus that develops will have cells with 47 chromosomes. Similarly, for a zygote with 45 chromosomes (monosomy), the fetus will have cells with 45 chromosomes.
Trisomy and monosomy are caused by non-disjunction.
Figure 1-3: Non-disjunction
Many of the common chromosomal disorders (chromosomal birth defects) are caused by non-disjunction and the resulting trisomy or monosomy of different chromosomes. Most fetuses with trisomy and monosomy are not capable of living and result in early spontaneous abortion.
The chromosomes which can result in an infant being born alive and surviving with trisomy are 13, 18, 21, X and Y. The common trisomies are:
Down syndrome, with trisomy of chromosome 21, is the commonest form of chromosomal birth defect.
The only chromosome that can be lost and result in a live born infant with monosomy is a sex chromosome X or Y. Therefore, the only monosomy seen is Turner syndrome (i.e. 45,X).
Down syndrome, with trisomy of chromosome 21, is the commonest form of chromosomal birth defect.
Figure 1-4: The karyotype of Down syndrome in a male with an extra chromosome 21 (trisomy 21)
In the normal zygote there are 46 chromosomes. The zygote then begins dividing by mitosis to form the embryo which contains many cells. This division of the one-celled zygote results in a doubling of cells to 2, 4, 8, 16, 32 cells and so on, with all the cells having 46 chromosomes. However, in mosaicism, an error occurs in the zygote. Early on in this dividing process one of the cells is involved in non-disjunction resulting in one cell having 47 chromosomes (trisomy) and the other cell only 45 chromosomes (monosomy). The monosomy cell usually dies but the trisomy cell may survive and divide. All future cells that come from it will be trisomy cells. Therefore, the embryo, fetus and infant that result will have some cells which are normal with 46 chromosomes and other cells which are abnormal with 47 chromosomes. This is called mosaicism (the presence of 2 different cell lines of the same genetic origin in a person).
As an example, if the non-disjunction was with chromosome 21 in a female then the newborn infant would have some cells of 46,XX and others of 47,XX +21 resulting in mosaic Down syndrome (46,XX/47,XX +21). Mosaicism causes 1 to 2% of the infants born with Down syndrome. People with Turner syndrome can also be mosaic (46,XX/45,X0).
In the formation of gametes (ova or sperms, if none of the chromosome pairs separate, then 46 chromosomes will go to one gamete and none to the other gamete. If the gamete with 46 chromosomes becomes fertilised with a normal gamete with 23 chromosomes, the resulting zygote will have an extra set of chromosomes, i.e. 69 chromosomes (69,XXX or 69,XXY). This is called triploidy. It is also possible to have more than one extra set of chromosomes (polyploidy). Embryos with polyploidy usually abort spontaneously early in pregnancy. On the rare occasion when a triploidy (three sets of chromosomes) infant is born, it is very abnormal and either dies before delivery or very early in the neonatal period.
Translocation occurs when a piece of one chromosome breaks off and joins (translocates) onto another chromosome. If in this process no genetic material is lost or gained, this is called a ‘balanced’ translocation and the person is clinically normal. However, if chromosome material is lost or gained then this is an ‘unbalanced translocation’ and the person will be abnormal because their genetic plan has lost or gained genetic material.
Persons with balanced translocations are at risk of passing on the abnormal chromosomes to their offspring, resulting in abnormal embryos with unbalanced translocations. This can be the cause of recurrent spontaneous abortions. If the embryo survives, the resulting infant will be abnormal. This risk varies according to the type of translocation.
This occurs when a piece of a chromosome, big or small, is missing. There are several recognised syndromes in which a known piece of chromosome is missing. These include:
A piece of one end of a chromosome may come off (deleted) making it sticky. This end then sticks to the other end making a ‘ring’ chromosome. Because genetic material is lost from the one end of the chromosome in the process, the person usually has a chromosome disorder, often associated with growth failure and intellectual disability.
The genetic material on chromosomes is divided up into smaller packages of DNA called genes. Like chromosomes, genes occur in pairs, one gene from each parent. Together, each pair of genes usually determines a single inherited function by giving a set of instructions to the cell, such as a physical feature (e.g. hair colour) or a single biochemical product (e.g. production of a protein or an enzyme).
Genes make up the smallest parts of the genetic code. Children look like their parents because their genes are a mixture that is inherited from both mother and father. As this combination varies with each child, siblings look alike and yet have their differences. The only individuals with identical genes are identical twins.
A gene is a small section of a chromosome and controls a cell function. Genes occur in pairs, one being inherited from each parent.
On the chromosomes, a person’s genetic plan is coded (‘written’) in thousands of genes. Genes on the 22 autosomes and two X chromosomes always occur in pairs (alleles). One gene in each matching pair is inherited from the mother and the other gene in that pair is inherited from the father. Each pair of genes together codes for an inherited biochemical product (e.g. blood clotting factor) or physical feature (e.g. eye colour) and gives the cell an instruction to carry out a particular activity. If the structure of the gene is abnormal, the instruction will also be abnormal and this may be harmful to the individual. A birth defect that results from an abnormality in a gene is called a single gene defect.
A birth defect caused by an abnormality in a gene is called a single gene defect.
Almost all genes are normal and give the cell correct instructions. However, a gene can become abnormal by mutation. With a mutation, the DNA structure of a gene changes.
A mutation is a change in gene structure that can cause abnormal gene function and a birth defect.
Mutations are rare and may occur spontaneously or be caused by environmental factors, including radiation (solar radiation from the sun, nuclear radiation or excessive X-rays). These abnormal genes can be passed onto the next generation in the same way as normal genes are inherited. As a result, single gene defects are usually inherited (unlike chromosomal defects).
Single gene defects are usually inherited.
A gene may be either a dominant or a recessive gene. Both dominant and recessive genes may be normal or abnormal.
Genes can be either dominant or recessive.
In a pair of genes (alleles), the individual genes may be of different strengths, with the one being ‘stronger’ and the other being ‘weaker’. The ‘stronger’ gene dominates (overpowers) the ‘weaker’ gene. Therefore, the ‘stronger’ gene is called a dominant gene. The dominant gene controls the function of that gene pair (alleles).
A dominant gene controls the function of that gene pair.
If the dominant gene is abnormal, then the instructions sent from that gene pair will also be abnormal. As a result the cell may not function normally, causing a birth defect.
If the dominant gene is on one of the 22 autosomes, it is called an autosomal dominant gene. A clinical disorder caused by a mutation in an autosomal dominant gene is called an autosomal dominant disorder. These conditions may be mild or severe but usually are not lethal (otherwise they probably would not be passed on to the next generation). Males and females are equally affected by autosomal dominant disorders.
If either the father or mother has an autosomal dominant gene, there is a 50% chance of passing that gene on to each of their children. Both sons and daughter have an equal chance of inheriting an autosomal dominant gene.
There is a 50% chance of inheriting a dominant gene from a parent.
If the autosomal dominant gene causes an abnormality of structure or function, the genetic abnormality will be present in the parent with that gene, and also in each child that inherits that abnormal gene. Autosomal dominant disorders are, therefore, passed from one generation to the next. The clinical effect of the abnormal gene will usually be present in both parent and child.
While most autosomal dominant genes are inherited, an autosomal dominant gene may also appear in a person for the first time in a family as a result of a new mutation. That gene will not be present in either parent. Therefore, the parents will be normal but the child will have the disorder. However, the new mutated gene can be passed onto future generations in the same way as other autosomal dominant genes are inherited.
Figure 1-5: The pattern of autosomal dominant inheritance. There is a 50% chance that the autosomal dominant gene (e.g. D) will be passed from the affected parent to each child no matter whether a boy or girl.
If a dominant gene overpowers (suppresses) a ‘weaker’ gene, the weaker gene is called a recessive gene. The dominant gene will control the function of that pair of genes. As a result, the instructions sent to the cell will be that of the dominant gene only. Therefore, the recessive gene will have no control over the cell and its effect will be ‘hidden’ or suppressed.
A person is called a carrier if she/he carries a ‘hidden’ recessive gene. In a carrier the effect of an abnormal recessive gene is not seen and the individual appears normal.
A person who has both a dominant and a recessive gene (a carrier), is said to be heterozygous for that pair of genes. If both genes are the same (both genes are dominant or both recessive), the person is said to be homozygous for that pair of genes. Only if both genes are recessive will the recessive genes together control that function of the cells. They are able to do this as there is no dominant gene. Recessive genes may be normal (e.g. carry instructions for blue eyes) or abnormal (e.g. carry instructions for oculocutaneous albinism). If both recessive genes are abnormal, that function of the cell will also be abnormal. A clinically normal carrier has both a normal (dominant) and an abnormal (recessive) gene for that feature.
A recessive gene on an autosome is called an autosomal recessive gene.
If both parents are carriers (i.e. they are heterozygous) for the same recessive gene, their children will have a 25% chance of inheriting the recessive gene from both mother and father (i.e. the child will be homozygous). Their children will also have a 50% chance of inheriting a recessive gene from only one parent to become a carrier (i.e. heterozygous). Getting the same recessive gene from both parents is commoner if the parents are closely related, e.g. siblings, cousins or an uncle and a niece (intermarriage or a consanguineous relationship), as they may inherit the same recessive gene from a common ancestor (e.g. grandparent).
With autosomal recessive inheritance, the parents and grandparents are usually normal and do not show the effect of the recessive gene. If a child inherits two abnormal autosomal recessive genes (i.e. one from each parent), they will have an autosomal recessive disorder. The risk of an autosomal recessive disorder is much higher if the parents are closely related (consanguineous).
The majority of single gene defects are autosomal recessive. Males and females are equally at risk of an autosomal recessive disorder.
If only one parent is heterozygous (a carrier), the children cannot be affected but they have a 50% risk of inheriting the recessive gene and, therefore, also being a carrier.
If both parents are carriers of a recessive gene, there is a 25% chance (1 in 4) that their child will inherit both recessive genes.
Figure 1-6: The pattern of autosomal recessive inheritance. If both parents are heterozygous for a recessive gene (e.g. r), there is a 25% chance that a child will be homozygous and a 50% chance that a child will also be heterozygous for that gene.
If a recessive gene is on an X chromosome, it is called an X-linked recessive gene (X-linked dominant genes and Y-linked genes are very rare).
X-linked recessive genes are inherited by girls in the same way as autosomal recessive genes. Girls have two X chromosomes and all the X-linked genes are in pairs. However, as the X and Y chromosomes are not identical (the Y chromosome is very short) the X-linked recessive genes in a male are not matched to a gene on the Y chromosome. Therefore, the X-linked gene, whether dominant or recessive, alone controls that cell function in males. As with autosomal recessive inheritance, a mother have a 50% chance (1 in 2) of passing her X-linked recessive gene to both her sons and daughters. However, it will only influence the function of the cell in her sons. It has no effect in her daughters as the gene is matched by a gene on the other X chromosome, inherited from the father.
Therefore, disorders caused by X-linked recessive genes are carried by females and affect males. Males have unaffected sons as they give them their Y and not their X chromosomes. However, there is a 100% chance that each daughter of an affected male will be a carrier.
Disorders caused by an X-linked recessive gene are called X-linked recessive disorders, e.g. colour blindness and haemophilia.
X-linked recessive disorders affect males and not females.
X-linked recessive genes are carried by mothers and affect 50% of their sons.
Figure 1-7: The pattern of X-linked recessive inheritance. There is a 50% chance that the recessive gene from the mother will be inherited by both sons and daughters. Only sons will be clinically affected as the X-linked recessive gene in daughters will be paired by a normal matching gene from the father.
The most common autosomal dominant disorders are:
The most common autosomal recessive disorders are:
Cystic fibrosis (the one autosomal recessive disorder that is common in people of European descent).
The most common X-linked recessive conditions are:
Some single gene disorders are more common in particular populations or regions, e.g. sickle cell anaemia in West Africa, cystic fibrosis in Europe, thalassaemia in Mediterranean countries, polydactyly in black South Africans. Most autosomal recessive conditions are found in, or come from, tropical countries. Cystic fibrosis is the one autosomal recessive disorder that is common in people of European descent.
Some conditions, such as polycystic kidneys, osteogenesis imperfecta, retinitis pigmentosa and mental retardation, may be inherited by more than one mode of inheritance, e.g. in some families as a dominant while in other families as a recessive disorder.
Duchenne muscular dystrophy
Fragile X syndrome
Glucose 6 phosphate dehydrogenase deficiency
Vitamin D resistant rickets
Sickle cell anaemia
Treacher Collins syndrome
Spinal muscular atrophy
Tay Sachs disease
These are birth defects that have a combined genetic and environmental cause. The environmental factor (or factors) is often not known. The person affected with a multifactorial birth defect inherits a combination of genes from their parents that places them at an increased risk for a birth defect. If that individual then experiences certain environmental factors, the result will be a multifactorial birth defect. Multifactorial birth defects, therefore, require both genetic and environmental factors before they present. Neither the genetic factor nor the environmental factor alone will cause the birth defect. The risk that another child of the same parents will be affected by a multifactorial birth defect is small (about 5%). The risk of recurrence increases if more than one family member is affected.
Multifactorial birth defects are the commonest form of birth defect and usually affect a single limb, organ or system. They often present in infancy or childhood as congenital malformations such as:
Multifactoral birth defects are common but usually involve a single limb, organ or system and have a low risk of recurrence.
A teratogen is a fetal environmental factor that can cause a birth defect. This is different from multifactorial birth defects as teratogens cause birth defects without an obvious genetic factor. Therefore the chromosomes and genes are normal in children with birth defects caused by a teratogen.
A teratogen can be a chemical substance like alcohol, an infection like the rubella virus (German measles) or a physical agent like X-rays. Teratogens interfere with normal development of the embryo usually early in pregnancy, but some can also damage the fetus later in pregnancy. If exposure to the teratogen is removed, there is little risk of a similar birth defect in a further child in that family.
The development of an infant from conception to birth is divided into three phases. The effect of teratogens is different in each of these phases:
The pre-implantation phase: (1–17 days post conception or two to four weeks after the start of the last menstrual period).
During this phase the fertilised egg (zygote) develops from one cell to a ball of many cells (the conceptus). The conceptus floats in a layer of fluid which carries it from the fallopian tube into the uterus. At about 17 days post conception (four weeks after the start of the last menstrual period) the conceptus begins to burrow into the lining of the uterus. Implantation and the development of the placenta and umbilical cord now begin.
Before implantation it is very difficult for a teratogen to get to the developing conceptus and damage it. In the unlikely event that a teratogen does damage the conceptus, it is so small and fragile that it would die. Implantation would not happen and the women would not even know she had conceived. Therefore, teratogens do NOT cause birth defects in the pre-implantation phase (1–17 days post conception or two to four weeks after the last menstrual period).
Teratogens do not cause birth defects during the pre-implantation phase of development.
The embryonic phase: (17–6 days post conception or 4–10 weeks after the start of the last menstrual period).
With implantation and the development of the placenta, the developing infants is now called an embryo. The embryo and mother are in very close contact and a teratogen can now move easily from the mother through the placenta to the embryo. During this phase the organs of the body are developing. They are very sensitive and are easily damaged by teratogens. Teratogens do the most damage in the embryonic phase. Structural birth defects that occur during the embryonic phase are called malformations, e.g. a cleft lip.
Teratogens cause the most damage in the embryonic phase of development from four to 10 weeks after the start of the last menstrual period.
The fetal phase: (six days after conception to birth or from 10 weeks after the start of the last menstrual period to delivery).
By six days after conception the embryo has turned into a fetus with fully formed organs. The fetus still needs to grow and mature before being born. Teratogens generally do little damage to the fetus in this phase of development, but there are some exceptions. For example, the fetal brain, which can be damaged more easily than other organs, can still be affected in this phase by some teratogens, particularly drugs like alcohol.
External forces can result in birth defects after the fetus is normally formed (i.e. it is not a malformation). The cause of this type of birth defects is called constraint. There are two types of birth defects due to constraint:
Constraint is the type of birth defect caused by local mechanical pressure in the uterus deforming or disrupting part of the fetus.
Birth defects may, therefore be divided into:
Placing a birth defect into one of these three categories helps to identify the probable cause and timing of the defect.
A newborn infant at a district hospital is recognised as having a birth defect. The midwife comments that she very rarely sees birth defects. The doctor does not know the cause of the birth defect.
It is an abnormality of function or structure in a person which is present from birth.
Because many birth defects are not recognised. A child may even die of a birth defect without the correct diagnosis being made. As a result, birth defects are commoner than they seem to be.
The birth prevalence of a birth defect is the number of infants born with that birth defect per 1000 liveborn infants. In contrast, the prevalence of a birth defect is the number of individuals with that defect per 1000 people in that population.
Birth defects may be caused by:
No. Birth defects due to teratogens and constraint are not due to genetic causes. Therefore, they usually do not recur in the same family.
An infant is brought to hospital with multiple abnormalities which were present at birth. The doctor thinks that the birth defects are due to a chromosomal abnormality. A blood sample is sent to a genetic laboratory. The report states that the infant has a trisomy.
Chromosomes are packages of DNA (a collection of genes) which makes up the genetic plan for the structure and functions of the body.
Yes. Chromosomal defects usually cause multiple abnormalities including dysmorphic features, growth and developmental delay and malformations.
With a trisomy the cells have three instead of two copies of a particular chromosome. For example, in Down syndrome due to trisomy, there are three instead of the normal two chromosomes 21.
Non-disjunction. During the formation of the gametes (egg or sperm), one gamete receives two chromosomes in error while the other gamete does not receive a chromosome (from that pair of chromosomes).
No. With translocation a piece of one chromosome may be moved onto another chromosome. If the gamete gets the chromosome with the extra piece but not the chromosome that has lost a piece, that gamete will have extra genetic material.
Parents with brown eyes have a son with blue eyes. The father asks the genetic nurse how brown-eyed parents can have a blue-eyed child.
A single pair of genes. The gene for brown eyes is a dominant gene while the gene for blue eyes in a recessive gene.
Yes. The colour of your eyes depends on the genes for eye colour carried by your parents.
Because both parents are heterozygous, i.e. they each have one gene for brown eyes (dominant) and another for blue eyes (recessive). If they both give their recessive gene (for blue eyes) to their child, that child will be homozygous for the blue-eyed gene and, therefore, have blue eyes.
25%. This is the chance of being homozygous (having both genes recessive) if your parents are heterozygous. If one or both parents have two dominant genes for brown eyes, all their children will have brown eyes.
No. Many recessive genes (such as eye colour) are normal. However, recessive genes may be abnormal and, therefore, cause a clinical disorder when the child is homozygous.
A young couple wants to get married. However, the man has a serious birth defect which has been diagnosed as an autosomal dominant disorder. They ask their general practitioner what the chances are that their children will inherit the problem. They mention that they are cousins.
It is a clinical problem caused by having an abnormal dominant gene on an autosome.
One of the 22 pairs of non-sex chromosomes. The X and Y chromosomes are not autosomes.
It is a clinical condition caused by an abnormal dominant gene. A dominant gene is a ‘strong’ gene that will overpower a recessive gene with which it is paired. The dominant gene will determine the effect that pair of genes has on the cell.
50%. Therefore, the risk of having the same birth defect (disorder) is also 50%.
This will not affect the risk of the children inheriting the autosomal dominant disorder. It would, however, increase the risk of both parents being carriers (heterozygous) for an abnormal recessive gene.
Healthy parents of six children plan to have one last child. They have three normal daughters and one normal son. However, their other two sons both have a similar birth defect. The mother’s sister also has a son with the same birth defect. They want to know what the risk is of the planned child having the birth defect that is common in the family.
The pattern of inheritance suggests a recessive gene (either autosomal or X-linked).
This may be due to chance. However, it strongly suggests an abnormal X-linked recessive gene defect. The fact that the mother’s sister also has an affected son indicates an abnormal gene carried by the females and affecting the males in the family.
Nil. However, she has a 50% chance of being a carrier.