Objectives
:
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A. Describe and illustrate autosomal inheritance.
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B. Describe and illustrate X-linked inheritance.
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C. Define and illustrate co-dominance, dominant, recessive, phenotype and genotype.
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D. Use Punnett square to determine offspring from a variety of matings.
Approximate time: one week.
Just how the information coded in chromosomes is passed on to dictate the traits to appear in a new human is the subject matter of heredity.
Two patterns of inheritance can be described:
autosomal inheritance
, the traits associated with the twenty-two chromosomes other than the sex chromosomes, and
X-linked inheritance
, the traits associated with the X chromosome.
Several terms that are used to describe genes and chromosomes are as follows. The corresponding forms of a gene pair are called a
alleles
; i.e. the gene for tasting PTC (phenylthiocarbamide (T) and the gene for non-tasters (t). For some characteristics there can be multiple alleles or factors.
Genotype
refers to the genetic composition of the organism or cell; i.e. Tt or tt.
Phenotype
refers to what is actually seen; i.e. taster or non-taster. A
homozygous
genotype is one in which the two alleles are the same; i.e. TT or tt;
heterozygous
is one in which the two alleles are different; i.e. Tt.
HUMAN CHROMOSOMES
(figure available in print form)
Figure 4
AUTOSOMAL INHERITANCE
(figure available in print form)
Figure 5
Genes are present on chromosomes in the nucleus. Chromosomes are paired just as the alleles are paired. The two members of a pair of chromosomes are known as
homologues
. The two alleles of the same gene are found on a particular spot or
locus
(location) on the chromosome; i.e. in a heterozygote (Tt) the allele T is found on a particular locus on a particular chromosome; at a corresponding locus on the homologous chromosome is found its allele for non-taster.
In regard to traits or characteristics such as tasting, the pattern of inheritance falls into two main categories. Those traits or genes for them that are fully or almost fully expressed, phenotypically that is, are known as
dominant
; i.e. taster (T). The dominant gene has the same expression in the heterozygous or homozygous state. Those genes carried on the autosomal chromosomes are classed with autosomal dominant inheritance.
Criteria for Autosomal Dominant Inheritance
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1. The trait is transmitted by an affected person to half his children on the average, though in an individual family wide discrepancy from the 1:1 ratio may be seen.
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2. The trait appears in every generation, with no skipping.
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3. Unaffected persons do not transmit the trait to their children.
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4. The occurrence and transmission of the trait are not influenced by sex; i.e. males and females are likely to have the trait and equally likely to transmit it.
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A trait that is transmitted as autosomal
recessive
is expressed only in persons who have the gene in a homozygous state; i.e. non-taster (tt).
Criteria for Autosomal Recessive Inheritance
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1. The trait characteristically appears only in sibs, not in their parents, offspring or other relatives.
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2. On the average, one-fourth of the sibs of the propositus are affected; in other words, the recurrence risk is one in four for each birth.
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3. The parents of the affected child may be consanguineous.
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4. Males and females are equally likely to be affected.
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Figure 5 illustrates the transmission of the gene for PTC tasting in the dominant and recessive conditions.
So far the discussion has concerned only the genes that are expressed as fully dominant or fully recessive. When both alleles are fully expressed in the heterozygote the genes are said to be
co-dominant
. Such an example is shown in the blood groups. A person of blood group AB has both A and B antigens on his red cells. The allelic genes A and B are therefore co-dominant. There are more than two alternative alleles in blood groups. Another allele exists for persons with type O blood and the gene 0 is recessive to genes A and B. This means that type 0 blood has a genotype of OO, type A blood is AA or AO, Type B can be BB or BO and type AB is AB. Figure 6 shows the possible combinations resulting from the mating of blood types. This information can sometimes be used to exclude paternity.
Genes on the X-chromosome are classed as
X-linked
and genes on the Y chromosome as Y-linked. There is only one suggested trait, “hairy ears”, linked to the Y chromosome; it is found in adult males in India, Ceylon, Israel and aboriginal Australia. For practical purposes only the X-linked genes have any significance in expression.
Classical Hemophilia or Hemophilia A is an X-linked recessive, in which blood fails to clot normally because of an abnormality in the anti-hemophilic globulin or loss of factor VIII. A person with this disease shows a secondary severe arthritic condition as a result of bleeding into the joints. It occurs about 1 in 10,000 male births. The classic case is described among the descendants of Queen Victoria who was a carrier. There are many possibilities in the matings of persons with various genotypes. Two such matings are described in Figure 7. Other matings can be worked out and described in the classroom with a similar chart. It is interesting to note there is another form of hemophilia called Christmas disease or Hemophilia B that is controlled by another gene on the X-chromosome; in Hemophilia B there is a lack of factor IX in blood clotting.
Criteria for X-linked recessive inheritance
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1. The incidence of the trait is much higher in males than in females.
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2. The trait is passed from an affected man through all his daughters to half their sons.
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3. The trait is never transmitted directly from father to son.
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4. The trait may be transmitted through a series of carrier females, if so, the affected males in a kindred are related to one another through females.
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BLOOD GROUPS ABO
(figure available in print form)
Figure 6
X-LINKED INHERITANCE
(figure available in print form)
Figure 7
There are a few X-linked genes that are expressed as dominant; for example Vitamin D resistant rickets.
Criteria for X-linked Dominant Inheritance
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1. Affected males have no normal daughters and no affected sons.
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2. Affected females who are heterozygous transmit the condition to half their children of either sex. Affected females who are homozygous transmit the trait to all their children. Transmission by females follows the same pattern as an autosomal dominant. In other words, X-linked dominant inheritance cannot be distinguished from autosomal dominant inheritance by the progeny of affected males.
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3. Affected females are more common than affected males (twice as common if the disorder is rare.
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These are some general types of inheritance. There are other traits and disorders that can be caused by many factors known or unknown; diabetes is an example as well as spina bifida or anencephaly. How intelligence is affected by genetic factors and how it is affected by environmental influences is an age old debate. Certainly there are cases of mental retardation that can be traced back to genetic causes (as will be discussed later) but when one speaks about intelligence in general it has been stated “ . . . intelligence is a complex phenotype that develops under the influence of genes and the experiences of a lifetime.”
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One tool used by geneticists to study the inheritance of traits or characteristics is what is known as the pedigree chart in which family data can be summarized. This consists of designated symbols with connecting lines to show the transmission of traits. Examples of such charts are available in many biology textbooks and can be used to trace family traits with students in the classroom.
Table I summarizes some of the inherited human characteristics and the patterns of inheritance. Crosses of matings showing these traits can be worked out in the classroom using a chart similar to the one used to illustrate autosomal inheritance and X-linked inheritance etc. (see Appendix 1).
