Unit Three. The Continuity of Life
10. Foundations of Genetics
To study human heredity, scientists look at the results of crosses that have already been made. They study family trees, or pedigrees, to identify which relatives exhibit a trait. Then they can often determine whether the gene producing the trait is sex-linked (that is, located on the X chromosome) or autosomal, and whether the expression of the trait is dominant or recessive. Frequently the pedigree will also help an investigator infer which individuals in a family are homozygous and which are heterozygous for the allele specifying the trait.
Analyzing a Pedigree for Albinism
Albino individuals lack all pigmentation; their hair and skin are completely white. In the United States about 1 in 38,000 Caucasians and 1 in 22,000 African-Americans are albino. In the pedigree of albinism among a family of Hopi Indians presented in figure 10.28, each symbol represents one individual in the family history, with the circles representing females and the squares, males. In this pedigree, individuals that exhibit a trait being studied—in this case, albinism—are indicated by solid symbols; heterozygote “carriers” exhibiting normal phenotypes are indicated by half-filled symbols. Marriages are represented by horizontal lines connecting a circle and a square, from which a cluster of vertical lines descend indicating the children, arranged from left to right in order of their birth.
Figure 10.28. A pedigree of albinism.
In the photo, one of three girls from a Hopi Indian family (the left-most family in generation IV of the pedigree) is albino. The pedigree shows the inheritance of the gene causing albinism in this family, with the solid blue symbols indicating persons who are albino.
To analyze this pedigree of albinism, a geneticist traditionally asks three questions:
1. Is albinism sex-linked or autosomal? If the trait is sex- linked, it is usually seen only in males; if it is autosomal, it appears in both sexes fairly equally. In the pedigree below, the proportion of affected males (4 of 12, or 33%) is reasonably similar to the proportion of affected females (8 of 19, or 42%). (When counting numbers of affected individuals in a pedigree, exclude the parents in generation I, as well as any “outsiders” who marry into the family.) From this result, it is reasonable to conclude the trait is autosomal.
2. Is albinism dominant or recessive? If the trait is dominant, every albino child will have an albino parent. If recessive, however, an albino child’s parents can appear normal, since both parents may be heterozygous “carriers.” In the pedigree below, parents of most of the albino children do not exhibit the trait, which indicates that albinism is recessive. Four children in one family do have albino parents. The allele is very common among the Hopi Indians, from which this pedigree was derived, and thus homozygous individuals such as these albino parents are present in the Hopis in sufficient numbers that they sometimes marry. In this family, both parents are albino and all four children are albino, which is consistent with the finding that the trait albinism is recessive, with both parents homozygous for the allele.
3. Is the albinism trait determined by a single gene, or by several? If the trait is determined by a single gene, then a ratio of 3:1 (normal to albino) offspring should be born to heterozygous parents (indicated by half- filled symbols), reflecting Mendelian segregation in a cross. Thus about 25% of these children should be albino. But if the trait is determined by several genes, albinism would only be present in a few percent. In this pedigree, 8 of 24 children born to heterozygotes exhibit albinism, or 33%, strongly suggesting that only one gene is segregating in these crosses.
The albinism pedigree analysis you have just examined indicates that albinism is an autosomal recessive trait controlled by a single gene. The inheritance of other human traits is studied in a similar way, although sometimes with different results. As an example, let us analyze a different trait. Red-green color blindness is an infrequent, although not rare, inherited trait in humans, affecting 5% to 9% of males. Color blindness is a group of eye disorders in which a person is not able to distinguish certain colors or shades of colors. It doesn’t mean that they see only in black and white, but rather that they see colors but some different colors look the same to them. Special types of cells in the retina of the eye detect different colors of light and different shades. Recall from the discussion of the electromagnetic spectrum in chapter 6 that visible light contains different wavelengths of photons that appear as the spectrum of visible light shown here:
Our eyes contain three types of color receptors: one absorbs red light, one green light, and a third absorbs blue light. People with red-green color blindness have deficiencies in their ability to detect red and green light as being different, and so these colors appear the same to them. Test samples called Ishihara plates are used to determine if a person is color blind. The test plates contain different colored dots arranged to reveal a shape, often a number. People with normal vision are able to see the number while a person who is color blind for those colors is not able to see it. An Ishihara test for red-green color blindness is shown in figure 10.29.
Figure 10.29. Pedigree of color blindness.
Individuals who are red-green color blind cannot see the number, as all the dots appear the same color. The pedigree traces red-green color blindness through four generations of a family.
Like albinism, a pedigree can be used to reveal the pattern of inheritance of color blindness. In the pedigree shown below, a red-green color blind man has five children with a woman who is heterozygous for the allele. Again, the solid- color symbols indicate an affected individual, in this case red-green color blind. Half-filled symbols indicate a heterozygous individual who carries the trait but does not express it.
To analyze this pedigree, you ask the same three questions as before:
1. Is red-green color blindness sex-linked or autosomal? Of the five affected individuals, all are male. The trait is clearly sex-linked.
2. Is red-green color blindness dominant or recessive? If the trait is dominant, then every color-blind child should have a color-blind parent. In this pedigree, however, that is not true in any family after that of the original male. The trait is clearly recessive.
3. Is the red-green color blindness trait determined by a single gene? If it is, then children born to heterozygous parents should be color-blind in about 25% of cases, reflecting a 3:1 Mendelian segregation of the trait. In this pedigree, 4 of 14, or 28%, of the children of heterozygous parents are color blind, indicating that a single gene is segregating (do not count the five children of the generation I parents because the father in this case is homozygous for the trait).
The results of this pedigree indicate that color blindness is caused by a single sex-linked, recessive gene. This doesn’t mean that females can’t be color blind, but in order for a female to be color blind, both X chromosomes would have to carry the color blind gene, and this only occurs in 0.5% of females.
Key Learning Outcome 10.9. The study of family trees can often reveal if an inherited trait is caused by a single gene, if that gene is located on the X chromosome, and if its alleles are recessive.