Dominant and recessive illnesses occur with equal frequency in males and females. This is because the genes involved are located on autosomes, which are the same in both genders. Many physical traits, however, obviously do differ between the two genders. In addition, gender
dramatically affects the inheritance of certain traits and illnesses that have no obvious connection to sexual characteristics.
These sex-linked traits are controlled by genes located on the sex
chromosomes. Humans have 46 chromosomes, including 44 autosomes (nonsex chromosomes) and the two sex chromosomes, which can be either X or Y. The autosomes come in 22 homologous pairs, present in both males and females. Females also possess a homologous pair of X chromosomes, while males have one X chromosome and one Y chromosome (the master gene for “maleness” is located on the Y chromosome). All eggs have an X chromosome, so the sex of a child is determined at the time of fertilization by the type of sperm. If the fertilizing sperm carries an X chromosome, the child will be female; if it carries a Y chromosome, the child will be male.
The X chromosome is much larger than the tiny Y chromosome, and most of the genes on the X chromosome do not have a homologous counterpart on the Y.
Genes on autosomes will always be present in two copies: one inherited from the maternal parent, the other from the paternal parent. The traits controlled by such autosomal genes will be generally unaffected by gender and will follow Mendelian patterns of inheritance (with the exceptions noted in previous sections). In contrast, genes on the X chromosome (X-linked genes) are present in two copies in females but only one copy in males. Female offspring will inherit one copy of an X-linked gene from each parent, but male offspring must inherit the Y chromosome from their father and therefore always inherit only the maternal allele of any X-linked gene.
For example, color blindness and hemophilia are sex-linked disorders. The mutated gene that causes these disorders is recessive and exists on the X chromosome. In order for a female, who is XX, to have a phenotype that is color blind or hemophiliac, both of her parents have to have the recessive gene. But since males have only one X chromosome inherited from their mother, if their mother expresses the recessive mutation, that trait
will automatically be expressed in the male child’s phenotype, since the male has no other gene to assert dominance over the recessive mutation.
The pedigree shown below diagrams three generations of a hypothetical family affected by hemophilia A.
This pedigree demonstrates many of the characteristic features of X-linked recessive inheritance. Heterozygous females are carriers who do not
express the disease. In contrast, all males with the mutated allele will
express the disease; there are no male carriers. Affected males will transmit the mutated allele to none of their sons but to all of their daughters, who will then all be carriers. Heterozygous females will transmit the disease to one-half of their sons, and one-half of their daughters will be carriers.
Affected males generally have an unaffected father and a mother who is a carrier; 50 percent of their maternal uncles will have the disease
Review Questions
1. During which meiotic phase are sister chromatids separated?
(A) Prophase I (B) Metaphase I (C) Metaphase II (D) Anaphase I (E) Anaphase II
2. One cell that undergoes meiosis will yield how many independent cells?
(A) 1 (B) 2 (C) 3 (D) 4 (E) 5
3. In which generation of a monohybrid cross do all the individuals look the same?
(A) Parental (B) F1
(C) F2
(D) F3
(E) F4
4. In a monohybrid cross, the two parental genotypes are AA and aa. The genotype of all F1plants is Aa. If the F1 plants are crossed to get
an F1 generation, what percentage of the F2generation will have the dominant phenotype?
(A) 0%
(B) 25%
(C) 50%
(D) 75%
(E) 100%
5. You are performing a dihybrid cross with two traits encoded by two different genes. These genes are on two different chromosomes and follow the law of independent assortment. The two parental genotypes are AABB and aabb. The genotype of all F1 plants is AaBb. What phenotype ratio will you observe in the F2 generation?
(A) 8:4:2:1
(B) 1:1:1:1 (C) 4:2:2:1 (D) 1:2:2:1 (E) 9:3:3:1
6. A scientist is studying the inheritance of two traits: stem height and stem color.
Tall is dominant over short; brown is dominant over green. The scientist wants to do a test cross to determine the genotype of a tall, brown F1 hybrid
individual. What will be the height and color of the other plant he uses in this test cross?
(A) Tall and brown (B) Tall and green (C) Short and brown (D) Short and green
(E) It doesn’t matter; any of the above will work.
7. You are studying the inheritance of stem height and stem color. For height, the tall phenotype is dominant over short; for color, the brown phenotype is
dominant over green. You mate a purebred tall, brown plant to a purebred short, green plant and all of the F1 offspring are tall and brown. You then mate two of these F1 hybrid offspring together, and you count the following
phenotypes in their F2 offspring: 61 tall and brown, 1 tall and green, 2 short and brown, and 18 short and green. Which of the following is probably true?
(A) The two genes controlling these traits are sex linked.
(B) The two genes controlling these traits might be on different chromosomes.
(C) The two genes controlling these traits are linked with no crossing over.
(D) The two genes controlling these traits are nearby on the same chromosome.
(E) The two genes controlling these traits are far apart on the same chromosome.
8. Which of the following is usually NOT true of an autosomal recessive disease?
(A) Most people with the disease have parents who do not have the disease.
(B) An equal number of males and females get the disease.
(C) If both parents have the disease, 50% of their offspring will have the disease.
(D) If both parents are heterozygous, 75% of their children will not get the disease.
(E) The genes for the disease are not located on the sex chromosomes.
9. A woman who is a heterozygous carrier of the hemophilia allele marries a man who has hemophilia. What percentage of their female children would you expect to have hemophilia?
(A) 0%
(B) 25%
(C) 50%
(D) 75%
(E) 100%
10. A boy with red-green color blindness (an X-linked recessive condition) has a color-blind father and a mother who is not color blind. From which parent did the boy inherit his color blindness?
(A) No way to know—it could have been either one.
(B) His father (C) His mother
(D) It had to come partly from both his mother and father.
(E) It had nothing to do with either his mother or father.