Biological Sciences
Sex-Linked Traits
Sex-linked traits are genetic characteristics that are determined by genes located on the sex chromosomes, particularly the X chromosome. In humans, these traits are often associated with X-linked inheritance, where the gene responsible for the trait is located on the X chromosome. This can result in different patterns of inheritance and expression of traits between males and females.
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7 Key excerpts on "Sex-Linked Traits"
eBook - PDF- Tomar, Arun Kumar(Authors)
- 2021(Publication Date)
- Daya Publishing House(Publisher)
Such genes are called as partially sex linked . In contrast to this, the genes located on non-homologous parts of X chromosome and inherit together due to no crossing over between non-This ebook is exclusively for this university only. Cannot be resold/distributed. homologous parts are called completely sex-linked genes . The genes for haulandric inheritance are completely sex-linked because of their presence on differential parts of chromosome having no homologous genes. Thus, the Y-linked genes inherit along Y chromosome and hence expressed only in males. Therefore, the Y linked (haulandric) genes and the sex-linked (X-linked) genes present on non-homologous part (of X chromosome) are known as sex-linked genes and their inheritance is called as sex linkage . Characteristics of sex-linked recessive and dominant characters The sex linked genes show dominance-recessive relationship and the co-dominance among the alleles affecting a character. The dominance of alleles changes the inheritance pattern of the sex-linked characters. A sex linked recessive gene (character) expresses itself in the following way: (i) The incidence of sex linked recessive trait is more in males (q) than in females (q 2 ). The proportion of male recessive to female recessive is 1:q if the recessive gene is more. The red-green colour blindness in humans is due to sex linked recessive gene. When the sex linked recessive gene is very rare (low frequency), its frequency in females (q 2 ) will be too low as there will be hardly any homozygous recessive female at all and the recessive gene of the female will remain hidden in the heterozygous condition. The hemophilia is a sex linked recessive trait and it is extremely rare in women for two reasons- one is the low frequency of the gene and second is that homozygous females are not viable. (ii) The sex linked recessive traits express themselves showing the crisscross inheritance (To skip a generation) .
eBook - PDF- Kathleen A. Ireland(Author)
- 2018(Publication Date)
- Wiley(Publisher)
But half of her eggs will carry the defective gene, meaning her sons are in danger of being color-blind. Because the fer- tilizing sperm carries a Y chromosome, it cannot provide a second correctly formed copy of the damaged allele to overcome the defect, resulting in a color-blind male child. Despite these differences, inher- itance patterns for these so-called Sex-Linked Traits can be predicted using a simple Punnett square (named after a fascinating British FIGURE 21.6 Hair patterns Note that the uppercase and lowercase conventions are not used here because one trait is not dominant over the other. m-imagephotography / Getty Images Giulia Cattelan / EyeEm / Getty Images PHENOTYPES GENOTYPES HH (curly) HH (wavy) HH (straight) Digital Vision / Getty Images 474 CHAPTER 21 Inheritance, Genetics, and Molecular Biology 21.3 Genetic Theory Is Put to Practical Use LEARNING OBJECTIVES 1. Explain the information in a pedigree chart. 2. Define Sex-Linked Traits. 3. Describe chromosomal disorders and genetic counseling. 4. Compare the values and costs associated with prenatal testing. Couples often request genetic counseling before they choose to con- ceive. Genetic counseling is the practice of predicting the potential combinations of alleles two individuals may produce. If there is a family history of congenital disease, or if the potential parents feel they are at risk of carrying a detrimental recessive allele, genetic counseling can help alleviate their fears. Knowing the probability of having a child with a genetic anomaly can help couples decide whether to conceive. Pedigree Charts Trace Traits Through Families Pedigree charts are symbolic representations of genetic transmission of phenotypic traits through families. Using a pedigree chart like the one shown in Figure 21.8, researchers can trace the pathway of a dis- ease through families, and characteristics of its transmission can be deduced.
eBook - PDF- D. Peter Snustad, Michael J. Simmons(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
GENES ON BOTH THE X AND Y CHROMOSOMES Some genes are present on both the X and Y chromosomes, mostly near the ends of the short arms (see Figure 5.2). Alleles of these genes do not follow a distinct X- or Y-linked pattern of inheritance. Instead, they are transmitted from mothers and fathers to sons and daughters alike, mimicking the inheritance of an autosomal gene. Such genes are therefore called pseudoautosomal genes. In males, the regions that contain these genes seem to mediate pairing between the X and Y chromosomes. In the animal kingdom, sex is perhaps the most conspicu- ous phenotype. Animals with distinct males and females are sexually dimorphic. Sometimes this dimorphism is established by environmental factors. In one species of turtles, for example, sex is determined by temperature. Eggs that have been incubated above 30°C hatch Sex Chromosomes and Sex Determination In some organisms, chromosomes—in particular, the sex chromosomes—determine male and female phenotypes. In this pedigree, II-1 is affected with X-linked hemophilia. If III-1 and III-2 have a child, what is the risk that the child will have hemophilia? Calculating the Risk for Hemophilia Solve It! I II 1 2 1 2 3 2 1 4 III ▶ To see the solution to this problem, visit the Student Companion site. ◾ FIGURE 5.9 Analysis of a pedigree showing the segregation of X-linked color blindness. I II III IV V Key: 1 2 1 2 1 2 1 2 3 4 1 3 4 5 3 4 5 Color blind Known carrier Y chromosome is transmitted with probability 1 /2. Y chromosome is transmitted with probability 1 /2. Mutant allele is transmitted with probability 1 /2. If a carrier, mutant allele is transmitted with probability 1 /2. Mutant allele is transmitted with probability 1 /2. P(IV-4 is color blind) = 1 /2 x 1 /2 = 1 /4 P(V-1 is color blind) = 1 /2 x 1 /2 x 1 /2 = 1 /8 Disorders such as hemophilia and color blindness, which are caused by recessive X-linked mutations, are more common in males than in females.
eBook - PDFOur Voices
Psychology of Women
- Elizabeth A. Rider(Author)
- 2015(Publication Date)
- Wiley(Publisher)
Notice also that the number of males born is not as high as the number conceived, which indicates that more males are lost during pregnancy. This loss is usually due to spontaneous miscarriage related to ge- netic disorders. Genetic factors also account, in part, for the changes in the proportion of males to females throughout the life span. However, other fac- tors also contribute to men’s greater physical vulnerability relative to women, including the male sex role (e.g., the type of work men are expected to do) and hormones (e.g., estrogens may offer protective value against some diseases). Genetic Complications Numerous genetic complications related to the sex chromosomes exist. For example, in sex-linked inheritance, traits are determined by genes located on one of the sex chromosomes, usually always the X. Genes present on the meiosis cell division that results in daughter cells with half the num- ber of chromosomes as the mother (original) cell sex-linked inheritance pattern of inheritance in which genes on the sex chromosomes influence traits. Typically, women are carriers of sex- linked traits, but do not show the traits in their phenotype. 94 ❘ CHAPTER 4 HOW BIOLOGY AFFECTS GENDER X chromosome may, when combined with the Y chromosome, cause specific sex-linked disorders that are typically passed from mothers to sons. Males are more vulnerable to sex-linked disorders than females because they don’t have a healthy X to dominate or “counteract” the effects of the X chromo- some with the disorder-producing gene. Females, though, express these re- cessive disorders only when they get an X carrying the gene for the disorder from both their mother and father. About 200 disorders are X-linked, in- cluding color blindness, some kinds of night blindness, hemophilia, and Duchenne’s muscular dystrophy. Perhaps females are actually the “stronger sex” due to their two X chromosomes, which protect them from numerous sex-linked disorders.
eBook - PDF- Cecie Starr, Beverly McMillan(Authors)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
X-linked gene Any of the genes on an X chromosome. Y chromosome The male sex chromosome. Y-linked gene Any of the genes on a Y chromosome. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. CHROMOSOMES AND HUMAN GENETICS 389 HOW DO THE FEATURES AND FUNCTIONS OF THE X AND Y SEX CHROMOSOMES COMPARE? • The X chromosome, the female sex chromosome, is much larger and carries more genes than the Y chromosome—the male sex chromosome. • A person’s sex is determined by the father’s sperm, which can carry either an X chromosome or a Y chromosome. XY embryos develop as males, and XX embryos as females. • In females, one of the two X chromosomes is inactivated soon after embryonic development begins. • Sex chromosomes do not carry genes for sex-influenced traits, such as pattern baldness. • Genes that govern secondary sexual characteristics are on the X chromosome. Male and female sex hormones determine which ones are expressed in a given individual. • Sex-influenced traits are governed by genes on autosomes but are expressed differently in males and females. TAKE-HOME MESSAGE Some genes are expressed differently in males and females You may have noticed that many more men than women have pattern baldness. This common form of hair loss in men is an example of a sex-influenced trait. Such traits appear much more often in one sex than in the other, or else the phe-notype differs depending on whether the person is male or female. Genes for sex-influenced traits are on autosomes, not on sex chromosomes.
- David L. Hull, Michael Ruse(Authors)
- 2007(Publication Date)
- Cambridge University Press(Publisher)
The most recent large survey of sexual behavior, conducted in the same country where the family data were generated (United States) and using the same cri- teria (self-identification as gay), produced rates of homosexuality of 2.4 percent in men and 1.4 percent in women (Laumann, Michael, and Gagnon 1994). The fact that the rate of homosexuality in rela- tives with a gay family member is higher than the population base rate suggests evidence for the familiality of both male and female sexual orientation. Family studies are also useful for determining whether the pat- tern of transmission of a trait is autosomal or sex linked. Humans have twenty-three pairs of chromosomes, twenty-two of them classified as autosomes and one pair classified as sex chromosomes. Sex-Linked Traits are those that have predisposing genes transmitted on the X or Y chromosomes, and they have distinct patterns of inheritance. Since the Y chromosome is small and is believed not to contain many genes, most Sex-Linked Traits are influenced by genes christopher horvath 294 on the X chromosome. Males inherit their Y from their father and their X from their mother; thus maternal transmission of a trait is the strongest evidence that a gene is located on the X chromosome. The first family study to examine patterns of transmission found gay men had more gay uncles and cousins on the maternal side than the paternal side of their family (i.e., maternal transmission; Hamer et al. 1993). As previously mentioned, such a pattern of results suggests that a gene associated with male homosexuality may be located on the X chromosome. One subsequent study replicated the findings of maternal transmission (Rice et al. 1999), while two others have not (Bailey et al. 1999, McKnight and Malcolm 2000). More research will be needed to clarify this incongruity.
Available until 27 Jan |Learn moreGenes and DNA
A Beginner's Guide to Genetics and Its Applications
- Charlotte K. Omoto(Author)
- 2004(Publication Date)
- Columbia University Press(Publisher)
A and B show the steps in determining the genotype of normal parents who have a child with sickle-cell anemia, a recessive disease. A is the normal form of the gene and a its abnormal form. C and D show the steps in determining the probability of having a child with Marfan syndrome, a dominant disease. Here, M is the abnormal form of the gene and m its normal form. A. A partially filled-in Punnett square showing only the child with sickle-cell anemia (genotype aa) in one of the four offspring squares. B. A partially filled-in Punnett square that shows that both parents must be heterozygous (Aa) if they are normal and have a child with sickle-cell anemia. C. A partially filled-in Punnett square showing only the heterozygous parent with Marfan syndrome (Mm) and the normal parent who must be homozygous normal (mm) because Marfan syndrome is a dominant trait. D) A completely filled-in Punnett square that shows a 50 percent chance for this couple to have an affected child (Mm). Another Sex-Influenced Trait: Male Pattern Baldness A more dramatic example of a sex-influenced trait is male pattern baldness. Because mostly men are affected, one might guess that it is a sex-linked trait. A classic case of male pattern baldness appears in President Adams’s family. The second U.S. president, John Adams, and his son John Quincy Adams, the sixth president, as well as the latter’s son and grandson, all had male pattern baldness. If this trait was on the X chromosome, as figure 3.2 shows, a father could not pass the trait onto his sons, meaning they must have received the trait from their mothers. However, it would be quite a coincidence if all four of the mothers of the individuals listed above were carriers of male pattern baldness. Actually, male pattern baldness is also a sex-influenced trait. The difference in the appearance of the trait is due to the hormonal differences between men and women
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