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Sex-linked, Mitochondrial Inheritance (Learning Objectives) • Explain how gender is determined in mammals. • Define X- or Y-linked genes. How does the location of a gene on the X chromosome affect its gender-related transmission? • Use a Punnett square to determine the probability of passing of an Xlinked gene and the phenotype to girls or boys based on the genotypes of the parents. • Explain the difference between sex-limited traits and sex-influenced traits. • Explain X-inactivation and why it exists only in cells of females. • Explain the functions of the Y chromosome gene and the pattern of inheritance of Y-linked traits. • Explain the pattern of inheritance of genes present on the mitochondrial DNA why? Sex determination in Mammals: the X-Y system Karyotype designation: 46, XY (male) 46, XX (female) (homogametic) Males are heterogametic Germ cells in testes (XY) produce sperms with X: 50% Y: 50% Females are homogametic Germ cells in ovaries (XX) produce only X eggs Sex Determination in Humans Figure 6.6 Figure 6.6 X and Y Chromosomes X chromosome - Contains > 1,500 genes - Larger than the Y chromosome - Acts as a homolog to Y in males Y chromosome - Contains 231 genes - Many repeated DNA segments Figure 6.2 Anatomy of the Y Chromosome Pseudoautosomal regions (PAR1 and PAR2) - 5% of the chromosome - Contains genes shared with X chromosome Male specific region (MSY) - 95% of the chromosome - Contains 22 male-specific genes including SRY and AZF (needed for sperm production) Figure 6.3 The inheritance of genes of X chromosome • males have only a single X chromosome • almost all the genes on the X have no counterpart on the Y • Genes are described as sex-linked or Xlinked. X-linked Traits Possible genotypes X+X+ − Homozyogus wild-type female X+Xm − Heterozygous female carrier XmXm − Homozygous mutant female X+Y − Hemizygous wild-type male XmY− Hemizygous mutant male X-linked Recessive Traits Examples: - Ichthyosis = Deficiency of an enzyme that removes cholesterol from skin - Color-blindness = Inability to see red and green colors http://www.biology.arizona.edu/human_bio/problem_sets/color_bli ndness/color_blindness.html - Hemophilia = Disorder of blood-clotting http://www.ygyh.org Ichthyosis Figure 6.7 Figure 6.7 X-linked Dominant Traits Congenital generalized hypertrichosis Figure 6.10 X Inactivation Females have two alleles for X chromosome genes but males have only one In mammals, X inactivation balances this inequality and one X chromosome is randomly inactivated in each cell The inactivated X chromosome is called a Barr body X Inactivation X inactivation occurs early in prenatal development It is an example of an epigenetic change The XIST gene on the inactive X encodes an RNA that binds to and inactivates the X chromosome Figure 6.11 Figure 6.12 X Inactivation A female that expresses the phenotype corresponding to an X-linked gene is a manifesting heterozygote (calico cats) Figure 6.12 Y-linked genes The Y chromosome in males has 231 gene genes whose protein products are involved in: a. control of changing sex of the fetus from female to male b. development of male testes c. male fertility http://ghr.nlm.nih.gov/chromosome=Y Gender • Maleness or femaleness is determined at conception • Another level of sexual identity comes from the control that hormones exert on development • Finally, both psychological and sociological components influence sexual feelings 19 During the fifth week of prenatal development, all embryos develop two sets of: - Unspecialized (indifferent) gonads - Reproductive ducts: Müllerian (female-specific) & Wolffian (male-specific) An embryo develops as a male or female based on the absence or presence of the Y chromosome - Specifically the SRY gene (sex-determining region of the Y chromosome) Figure 6.1 Figure 6.1 SRY Gene • • • • Encodes a transcription factor protein Controls the expression of other genes Stimulates male development Developing testes secrete anti-Mullerian hormone and destroy female structures • Testosterone and dihydrotesterone (DHT) hormones are secreted and stimulate male structures Abnormalities in Sexual Development • Androgen insensitivity syndrome = Lack of androgen receptors • Pseudohermaphroditism = Presence of male and female structures at different stages of life – 5-alpha reductase deficiency = Absence of DHT – Congenital adrenal hyperplasia = High levels of androgens 22 Figure 6.4 Autosomal Recessive Autosomal Recessive Precocious puberty in males and females XY females (x-linked) 23 Homosexuality • Person’s phenotype and genotype are in-consistent – Physical attraction is toward members of the same sex • Homosexuality has been seen in all cultures for thousands of years – Documented in 500 animal species 24 Homosexuality • Evidence suggests a complex input from both genes and the environment – Identical twins are more likely to be homosexual than members of fraternal twin pairs – Genetic markers were identified on the X chromosome more often identical among pairs of homosexual brothers 25 Sex Ratios • The proportion of males to females in a human population • • • • Calculated by # of males / # of females multiplied by 1,000 Primary sex ratio – At conception Secondary sex ratio – At birth Tertiary sex ratio – At maturity • Sex ratios can change markedly with age • Reflects medical conditions and environment factors 26 Sex-Limited Traits Traits that affect a structure or function occurring only in one sex The gene may be autosomal or X-linked Examples: - Beard growth - Milk production - Preeclampsia in pregnancy Sex-Influenced Traits Traits in which the phenotype expressed by a heterozygote is influenced by sex Allele is dominant in one sex but recessive in the other The gene may be autosomal or X-linked Example: - Pattern baldness in humans (autosomal) - A heterozygous male is bald, but a heterozygous female is not Genomic Imprinting The phenotype of an individual differs depending on the gene’s parental origin Genes are imprinted by an epigenetic event: DNA methylation - Methyl (CH3) groups bind to DNA and suppress gene expression in a pattern determined by the individual’s sex Genetic Imprinting • An inheritance process of gene expression • Only one allele of imprinted genes is expressed: mono-allelic expression (either paternal or maternal allele) • < 1% of the human genes are imprinted • Found in mammals Genetic Imprinting • Dynamic process: erase and reestablished between generations • Imprint is erased in germline cells and reestablished. – In testes, parental imprint is reset – In ovaries, maternal imprint is reset • Passed on to next generation Imprints are erased during meiosis - Then reinstituted according to the sex of the individual Figure 6.13 Mitochondrion • Organelle providing cellular energy • Contains small circular DNA called mtDNA - 37 genes without non-coding sequences • No crossing over and little DNA repair • High exposure to free radicals • Mutation rate is greater than nuclear DNA • A cell typically has thousands of mitochondria, and each has numerous copies of its “minichromosome” Figure 5.8 Mitochondrion • Mitochondrial genes are transmitted from mother to all of her offspring Figure 5.7 Mitochondrial Disorders Mitochondrial genes encode proteins that participate in protein synthesis and energy production Several diseases result from mutations in mtDNA maternally inherited Examples: - Mitochondrial myopathies – Weak and flaccid muscles - Leber optical atrophy – Impaired vision Ooplasmic transfer technique can enable woman to avoid transmitting a mitochondrial disorder Heteroplasmy • The mtDNA genome sequence may not the same in all mitochondria • The phenotype reflects the proportion of mitochondria bearing the mutation Figure 5.9