Sex%20Chromosomes

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Sex Chromosomes
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Sex Chromosomes
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X-linked Traits

• Possible genotypes
• XY ? Hemizygous wild type male
• XmY? Hemizygous mutant male
• XX ? Homozyogus wild female
• XXm ? Heterozygous female carrier
• XmXm ? Homozygous mutant female

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X-linked Recessive Inheritance

• Always expressed in hemizygous males
• Female homozygotes show the trait but female
  heterozygotes do not
• Affected males Inherited from affected or
  heterozygous mother
• Affected females affected fathers and affected
  or heterozygous mothers

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X-linked Dominant Inheritance

• Expressed with one copy
• Males are often more severely affected
• Typically associated with miscarriage or
  lethality in males
• Passed from father to all his daughters but none
  of his sons

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X-linked Dominant Inheritance Congenital
Generalized Hypertrichosis
Figure 6.10
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Sex-limited traits

• A trait that affects a structure or function of
  the body that is present in only one of the
  sexes.
• May be X-linked or autosomal
• Example A gene affecting milk production will
  not have an effect in males. However, males can
  carry and pass on the trait.
• Why do men have nipples anyway???

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Sex-influenced traits

• An allele is dominant in one sex but recessive in
  the other sex.
• May be X-linked or autosomal
• Due to hormonal interactions
• Men have testosterone
• Women have estrogen

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X-inactivation

• Females turn off one of their X chromosomes in
  each cell
• In order to be more equal to males who only have
  one X chromosome
• The X chromosome turned off in each cell is random

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X Chromosome Inactivation
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Fur Color in Tortoiseshell Cats
Orange fur
Black fur
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Manifesting Heterozygote

• A carrier of an X-linked trait who expresses the
  phenotype
• If a female is heterozygous for a recessive
  trait, the dominant allele will usually mask the
  recessive allele
• Unless the dominant allele is on the X chromosome
  that was inactivated
• Some cells will express the trait and others will
  not, depending on which X chromosome is
  inactivated

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Multifactorial Traits

• Genes and the Environment

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Polygenic Traits

• A trait is influenced by more than one gene
• May be multifactorial (influenced by environment)

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Polygenic Traits are Continuously Varying

• Each gene in the polygenic trait contributes to
  the phenotype to a varying degree
• Example Height
• Polygenic (influenced by multiple genes)
• Continuous

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Pure Polygenic Trait -Eye Color
Figure 7.3

• The number of human eye color genes is unknown
• Analysis will probably reveal many genes
• Mice have more than 60 eye color genes

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Analyzing Multifactorial Traits

• Difficult, requires multiple techniques
• Use human genome sequences, population, and
  family studies
• The frequency in a specific population
  Empiric risk
• The amount of inheritance due to genes
  Heritability

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Separating Genes and Environment

• Dizygotic twins Shared environment
• and 50 of genes
• Monozygotic twins Identical genotype, and
• shared environment
• Twins raised apart Shared genotype but
• not environment
• Adopted individuals Shared environment
• but not genes

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Concordance

• The percentage of pairs in which both twins
  express the trait
• Used to determine heritability
• Assumes both types of twins share similar
  environments
• MZ twins often share more similar environments

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Review
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