sex-linked and mitochondrial inheritance (learning
TRANSCRIPT
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Sex-linked and 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 X-
linked 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.
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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
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Figure 6.1Figure 6.1
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)
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Sex determination in Mammals:the X-Y system
Karyotype designation: 46, XY (male)
46, XX (female) (homogametic)
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Males are heterogameticGerm cells in testes (XY) produce sperms with
X: 50% Y: 50%
Females are homogameticGerm cells in ovaries (XX) produce only
X eggs
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Figure 6.6
Sex Determination in Humans
Figure 6.6
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X and Y ChromosomesX 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
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Anatomy of the Y Chromosome
Figure 6.3
Pseudoautosomal regions (PAR1 and PAR2)- 5% of the chromosome- Contains genes shared with X chromosome
Male specific region (MSY) - 95% of the chromosome- Contains majority of genes including SRY and AZF (needed for sperm production)
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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
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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 X-
linked.
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X-linked Traits
Possible genotypesX+X+ −Homozyogus wild-type femaleX+Xm −Heterozygous female carrierXmXm −Homozygous mutant female
X+Y − Hemizygous wild-type maleXmY− Hemizygous mutant male
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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_blindness/color_blindness.html
- Hemophilia = Disorder of blood-clotting http://www.ygyh.org
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Figure 6.7Figure 6.7
Ichthyosis
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X-linked Dominant Traits
Congenital generalized hypertrichosis
Figure 6.10
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Sex-Limited TraitsTraits 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
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Sex-Influenced TraitsTraits in which the phenotype expressed by a
heterozygote is influenced by sexAllele is dominant in one sex but recessive in the
otherThe gene may be autosomal or X-linked
Example:- Pattern baldness in humans (autosomal)
- A heterozygous male is bald, but a heterozygous female is not
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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
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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
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Figure 6.12Figure 6.11
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X InactivationA female that expresses the phenotype
corresponding to an X-linked gene is a manifesting heterozygote (calico cats)
Figure 6.12
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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 testesc. male fertility
http://ghr.nlm.nih.gov/chromosome=Y
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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
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Imprints are erased during meiosis- Then reinstituted according to the sex of the individual
Figure 6.13
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Mitochondrion• Organelle providing cellular energy
• Contains small circular DNA called mtDNA- 37 genes without noncoding sequences
• No crossing over and little DNA repair
• High exposure to free radicals
• Mutation rate is greater than nuclear DNA
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• A cell typically has thousands of mitochondria, and each has numerous copies of its “mini-chromosome”
Figure 5.8
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Mitochondrion• Mitochondrial genes are transmitted from
mother to all of her offspring
Figure 5.7
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Mitochondrial DisordersMitochondrial genes encode proteins that participate in
protein synthesis and energy production
Several diseases result from mutations in mtDNAaternallyinherited
Examples:- Mitochondrial myopathies – Weak and flaccid muscles- Leber optical atrophy – Impaired vision
Ooplasmic transfer technique can enable woman to avoid transmitting a mitochondrial disorder
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Heteroplasmy
Figure 5.9
• The mtDNA genome sequence may not the same in all mitochondria
• The phenotype reflects the proportion of mitochondria bearing the mutation