Unit 23 SEX DETERMINATION AND INHERITANCE LEARNING OBJECTIVE: 1. To learn the difference in the types of sex chromosomes inherited by the human male and female. 2. To gain some appreciation for the concept of gene linkage. 3. To gain experience in working sex-linkage and sex-influenced problems. INTRODUCTION Most animals have two types of chromosomes: sex and autosomal (body). The patterns of inheritance examined in the previous unit involved interaction of alleles on autosomal chromosomes. Autosomal chromosomes are homologous in both sexes; whereas, sex chromosomes are homologous in the female but not in the male. Figure 23-1 is an electron micrograph of an X chromosome and a Y chromosome. Actually the chromosomes have duplicated so there are two chromatids connected by the centromere. Thus the single X chromosome is linear and not an X shape. A female gets 2 X chromosomes and a male gets 1 X chromosome and 1 Y chromosome. The Y chromosome does not carry many genes but it does carry the gene for maleness. If an individual gets 1 Y chromosome, he will be a male regardless of how many X chromosomes he gets. Figure 23-1. Showing the structural difference between a Y chromosome and an X chromosome. 292
Figure 23-2 is a human karyotype. A karyotype is the sum total of all the morphological characteristics of the chromosomes in a cell. The chromosomes are arranged according to size and position of the centromere. Figure 23-2. Karyotypes of a human female and human male. 293
LINKAGE Linkage refers to the fact that genes on the same chromosome are linked to one another. There are about 23,000 genes located on only 23 different chromosomes; therefore, each chromosome averages about 1,000 genes. During meiosis, homologous chromosomes twist around one another, cross over, and, upon separating, may connect to parts of the other homologous chromosome (Fig. 23-3). The closer two genes are to one another the better the chance that they will remain linked during meiosis. Sex linked or X-linked refers to those genes located on the X chromosome. Because a male only gets one gene for sex-linked traits, whichever allele he gets will be expressed. The male cannot be heterozygous. Figure 23-3. Illustration of a pair of homologous chromosomes and their duplicated chromatids lining up (synapsis) with crossing over occurring. Red-green color-blindness is a sex-linked trait. It is the result of a defective gene that, when normal, codes for the production of the green pigment present in one type of cone within the retina of the eye. Normal vision is dominant (D) and color-blindness is recessive (d). In working sex-linked problems one can represent the individuals as X D Y- X D X d or DY Dd Male female male female 294
Note that the genotype is represented by either using X and Y to represent the sex chromosomes and a superscript to represent the allele, or by using letters to represent the allele and using a "Y" to represent the Y chromosome which lacks the gene. The possible genotypes and phenotypes which can exist in this example of a sexlinked characteristic are: DD = Normal female Dd = Normal female (carrier of the trait) dd = Colorblind female DY = Normal male dy = Colorblind male Now, let us work a genetics problem to see how sex linked traits are inherited. If a heterozygous female (she has normal color vision) marries a normal vision male and they have children, the probability in the offspring is as follows: Dd DY represents the genotype of the female represents the genotype of the male Types of Gametes: D and Y D and d are the sperm (male gametes) are the eggs (female gametes) Female Gametes D d Male Gametes D DD Dd Y DY dy Thus the resulting offspring would be: 25% DD female with normal color vision 25% Dd female with normal color vision (carrier of colorblind gene) 25% DY male with normal color vision 25% dy male who is colorblind Note that all female offspring have normal color vision. There is a 50% chance that male offspring will be colorblind. Consider this: A female with normal color vision marries a male with colorblindness. They have a color-blind son. Did the son inherit his father s color-blindness? Why or why not? Explain. Now consider the television commercial that is selling a product to prevent hair loss. The actor is talking about using the product to retard the loss of his hair as a result 295
of male-pattern baldness. Male-pattern baldness is a sex-influenced trait. During the commercial the actor holds up a photograph of his balding father and states that he does not want to blame his father for his hair loss because he loves his father. Did the son inherit his father's baldness? Why or why not? In the case of a sex-influenced trait, the hormones have an affect (influence) on the expression of the genes. Pattern baldness is dominant in the male and recessive in the female. Female Genotype Male Normal hair HH Normal hair Normal hair Hh Pattern baldness Thin hair hh Pattern baldness In this unit we have examined how the sex of the offspring is determined by the father. Also, we have learned how the gene present on the sex chromosome that is inherited from the mother have a greater influence on the phenotype of male offspring compared to the influence on the phenotype of female offspring. In the next unit, we will continue our investigation of human genetics by examining a few of the more common genetic disorders that can be encountered. 296
Unit 23 OBJECTIVE QUESTIONS OVER SEX DETERMINATION AND INHERITANCE 1. The closer two genes are to one another on the same chromosome, the the chance that they will be separated during meiosis. (A) lesser (B) greater 2. For a sex-linked trait, (A) a male gets two genes and a female gets one (B) male gets one gene and a female gets two (C) both the male and the female get one gene each (D) both the male and the female each get two genes. 3. Sons get sex-linked diseases from (A) their mother (B) their father (C) both parents. 4. Hemophilia is a sex-linked trait. A normal male and female have a child with hemophilia. From this you know that the child is a (A) boy (B) girl (C) boy or a girl. 5. In the case of sex-linked traits, you would expect to find males than/as females with the trait. (A) more (B) less (C) the same number of 6. The sex of a human offspring is determined by the parent. (A) male (B) female (C) neither (D) both 7. Sex-linked characteristics such as hemophilia are passed from the parent to the male human offspring. (A) male (B) female (C) neither (D) both 8. Mammals, including humans, have chromosome(s). (A) autosomal (B) sex (C) polysomal (D) autosomal and sex 9. Females possess (A) 2 X chromosomes (B) 2 Y chromosomes (C) an X and a Y chromosome 10. Males possess (A) 2 X chromosomes (B) 2 Y chromosomes (C) an X and a Y chromosome 11. A photograph that shows the sum total of all the morphological characteristics of the chromosomes in a cell is called a (A) linotype (B) karyotype (C) genotype (D) phenotype 12. The term is used to refer to genes that are on the same chromosome. (A) pairing (B) linkage (C) locked (D) synaped 297
13. The Y chromosome contains genetic material than the X chromosome. (A) more (B) less (C) the same amount of 14. The gene for maleness is carried by the chromosome(s). (A) X (B) Y (C) autosomal (D) both the X and Y 15. Humans have different chromosomes (A) 5 (B) 13 (C) 23 (D) 53 16. The closer two genes are on a chromosome, the the chance that they will remain linked during meiosis. (A) greater (B) lesser DISCUSSION QUESTIONS OVER SEX DETERMINATION AND INHERITANCE 1. What is the reason that sex linked traits are often referred to as X-linked traits? 2. Explain what is meant by the statement In the case of sex-linked traits, there is no dominance or recessiveness in the male. 3. In cats, yellow coat is determined by the b allele and black coat is determined by the B allele. The heterozygous condition results in a color known as tortoise shell. These genes are sex linked. What kinds of offspring would be expected from a cross of a black male and a tortoise-shell female? What are the chances of getting a tortoiseshelled male? 4. If at one time there were only one sex, which sex would it be? Explain. 5. How could the second sex have arisen? 6. If you had a male karyotype and a female karyotype in front of you, how could you distinguish one from the other? 298
7. Does a female child get the same amount of genetic material from each parent? Explain. 8. Does a male child get the same amount of genetic material from each parent? Explain. 9. What is a karyotype? 10. The sex chromosomes in a human male are said to be heteromorphic. What does this mean? 11. What does it mean to say a trait is sex-linked? 12. Red-green color-blindness is a sex-linked disease. Two normal visioned parents have a color-blind child. Give the genotype of the three individuals and the sex of the child. sex father mother child 13. Mammals, including humans, have two general types of chromosomes: a. b. 299