genetic traits and sex in humans
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Genetic Traits and Sex in Humans
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Learning Objectives
• What is a sex chromosome? • Understand how sex is determined and
know some specific examples.• Be able to describe how X and Y
chromosomes apply to the determination of human sexes.
• Understand the inheritance patterns of sex-linked genetic diseases in humans.
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• As humans, we are very accustomed to considering others as male or female, girl or boy, or man and woman.
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• The majority of sexually reproducing organisms on earth are functionally both male and female at the same time.
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• In species that do produce male and female individuals, sex may be determined in a number of different ways.
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Bees (Insects)
• Males are haploid; females are diploid
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• In some cases, sex is determined by special chromosome, called a sex chromosome.
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Birds
• Males are ZZ; females are ZW
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Mammals
• Males are XY; females are XX
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Humans Follow the XY Sex Determination Scheme• Humans have 23 pairs of
chromosomes– 22 pairs of autosomes and
1 pair of sex chromosomes
• If the sperm is Y-carrying, the child will be a boy
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Sex-Linked Genetic Disorders
• Genes for non-sexual characters are also found on sex chromosomes.
• Genes that are carried on the X or Y chromosomes are said to be “sex-linked”
• Sex-linked traits on the Y chromosome are only passed from father to son.
• X-linked dominant traits work like autosomal dominant traits
• X-linked recessive traits are more interesting genetically
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Variations in Size
• The X chromosome is much larger than the Y chromosome.
• It contains 1000+ genes.
• The Y chromosome is much smaller and contains about 26 genes.
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X-Linked Recessive Traits
• The X Chromosome of a male can display a recessive trait because the Y chromosome has no corresponding gene.
• Such disorders are called “sex-linked” recessive
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Examples of X-Linked Recessive Traits• Colorblindness
• Hemophilia
• Duchenne’s Muscular Dystrophy
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Colorblindness
• Most types are caused by a gene located on the X chromosome
• Red green colorblindness is the most common type– Amongst Caucasians;
• 8% of males are colorblind• 1% of females are colorblind
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Test For Colorblindness
• Ishihara Test• Normal color vision
see an “8”• Colorblind people
don’t see anything
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Sex-linked Genetics
• We use an “X” to identify that this characteristic is carried on the X chromosome and a superscript “C” for the dominant (normal) trait or lowercase “c” for the colorblind trait.
• XC = normal vision allele• Xc = colorblind allele• Y = normal male chromosome (no allele for
colorblindness)
Example #1
• Show a cross between a normal vision woman and a colorblind man
XCXC XcY
XC Xc Y
XCXc XCY
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Example #2
XCXc XCY
XC Xc XC Y
XCXC XcY
XCY XcY
• Show a cross between a carrier woman and a normal vision man
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Hemophilia
• Aka “bleeder’s disease”
• Much rarer than colorblindness– 1 in 10,000 males– 1 in 100,000,000 females
• A protein antihemophilic factor (AHF) is missing
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Queen Victoria
• Acquired hemophilia through a mutation– Parents didn’t have it or
pass it on
• Passed it to 1 son and at least two daughters
• The daughters were married to nobility in France and Germany and it caused the loss of at least one heir to the throne!
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Duchenne’s Muscular Dystrophy• Progressive wasting of skeletal muscle
• Seldom survive past early adulthood
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Sex-Influenced Traits
• “a trait that is NOT located on the sex chromosomes but is influenced by gender”
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Male Pattern Baldness
All homozygous dominant show the disease (male and female)No homozygous recessive show the disease (male OR female)But if heterozygous, only males show the disease