X-Linked Genes and Inheritance

• X-linked genes are on the X chromosome and are present in both males and females.
• Both males and females can inherit X-linked traits as they both carry at least one X chromosome.
• Males only have one set of X-linked genes because they have only one X chromosome.
• Females have two sets of X-linked genes because they have two X chromosomes.

Normal Sex Chromosome Inheritance

• A mother passes on either an X chromosome to her child.
• A father passes on either an X or a Y chromosome to their child.
• A female child inherits an X chromosome from each parent.
• A male child inherits an X chromosome from the mother and a Y chromosome from the father.

X-Linked Recessive Alleles

• A recessive disease-causing allele on the X chromosome in females can be masked by a dominant allele on the other X chromosome.
• As males only have one X chromosome, a recessive allele on the X chromosome will be expressed in their phenotype.
• X-linked recessive diseases are more common in males than females.

Hemophilia

• Hemophilia is an X-linked disorder caused by a lack of a clotting factor, leading to severe bleeding.
• Hemophilia is mainly a recessive X-linked disorder, females with one healthy allele on the other X chromosome can be carriers.
• Most female carriers do not show symptoms because their healthy allele produces enough clotting factor.
• Males who inherit an X chromosome with the altered allele will not produce enough clotting factor and will suffer from hemophilia.
• The allele for a healthy clotting factor is denoted by XH, and the altered allele related to hemophilia is denoted by Xh.
• Women can have three possible genotypes (XHXH, XHXh, XhXh) while men can have only two (XHY, XhY).

Table 11.2 Summary of Possible Genotypes and Phenotypes of Healthy Versus Hemophilic Individuals

• A female with genotype ΧHXH has a healthy phenotype.
• A female with genotype XHXh is a carrier of the hemophilia allele
• A female with genotype XhXh suffers from hemophilia.
• A male with genotype XHY has a healthy phenotype.
• A male with genotype XhY suffers from hemophilia.
• Hemophilia is more common in males than females.

Color Blindness

• Color blindness is an X-linked condition characterized by the inability to distinguish certain colors.
• Red-green color blindness: an individual cannot distinguish between reds and greens.
• A gene on the X chromosome regulates different types of cone receptors in the retina to recognize blue, green, and red light.
• Red-green color-blind individuals carry a recessive altered allele on the X chromosome, resulting in the production of only two types of cone receptors , instead of three. Color blindness is more common in males than females because it is X-linked.

Genetic Probabilities

• Haploid gametes are produced through meiosis.
• Female gametes are eggs; male gametes are sperm.
• Fertilization: sperm and egg unite, the result is a diploid zygote.
• Zygote carries a combination of gene alleles from the sperm and egg.
• If a sperm carrying the IO allele fertilizes an egg carrying the IB allele, a zygote with the IOIB genotype for the ABO gene and blood group B will be produced.
• The specific type of cross, called a monohybrid cross, will be discussed.
• A monohybrid cross is a genetic cross between two organisms in which the inheritance of only one trait is being considered.

Crosses Between Homozygous Individuals

• Focus on the inheritance of fur color by studying a cross between two homozygous individuals.