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Incomplete and Co Dominance
& Sex Linked Traits
Mr. Fernandes
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Review of Mendel’s Principles
Genes are passed parents offspring;
get one allele from each parent
During Meiosis, the alleles for a gene
segregate from each other.
During Meiosis, genes independently
assort with each other.
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Exceptions to Mendel’s principles
Sometimes, there is no dominant or
recessive gene, or the trait is controlled
by many alleles or genes.
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Karyotype: a picture of chromosomes
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Body Cells vs. Sex Cells
Autosomes: the first 22 homologous pairs of chromosomes.
Autosomes are the same for both males and females.
Sex cells: Determine sex of offspring
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Is it Male or Female? Explain
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Sex chromosomes: determines the sex of the individual
The sex chromosomes are the 23rd pair of chromosomes.
XX =female
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DAD
Why?
All moms have the genotype XX. When egg cells are made, they will all carry a single X chromosome.
All dads have the genotype XY. When sperm cells are made, 50% will have an X chromosome and 50% will have a Y chromosome.
Therefore, males and females are born in roughly a 50:50 ratio.
Which parent determines the sex of an offspring?
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SEX-LINKED TRAITS
Those traits that are controlled by genes on the X or Y chromosomes.
NOTE: The Y chromosome is much smaller than the X chromosome and only contains a few genes. Most sex-linked traits are on the X chromosome.
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In humans, hemophilia is a sex-linked trait. Having
hemophilia is recessive (Xh) to being normal (XH). The
heterozygous female is called a carrier. Cross a carrier
female with a normal male.
__ XHXh __ X __ XHY ___ XH Xh
XH
Y
XH XH
XH Y Xh Y
XH Xh
male
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Now You Try
Cross a carrier female with a male with hemophilia.
__ XHXh __ X __ XhY ___
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In Incomplete Dominance, every genotype has its own
phenotype. (One allele not completely dominant over the
other.) Third phenotype that is a blending of the parental
traits. (2 alleles produce 3 phenotypes.)
Result: Heterozygous phenotype somewhere in
between homozygous phenotype.
Incomplete Dominance
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1. Incomplete Dominance
Examples:
Trait: Flower Color
Expressions: Red x
White Pink
RR= Red; RW= pink; WW=
white
straight hair, wavy, curly
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Incomplete Dominance
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In codominance, neither allele are dominant;
both are expressed. A cross between
organisms with two different phenotypes
produces offspring with has both phenotypes
of the parental traits shown.
Codominance
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2. Codominance Both alleles contribute to the phenotype.
Example: In come chickens
Black Chicken x White Speckled Chicken
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YOU tell me which type of
dominance…
Codominance!
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Type of
Dominance?
Incomplete
Dominance!
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Type of Dominance?
Incomplete Dominance!