albinism is a disorder caused by a recessive allele. about 1 out of every 75 people carry the albino...
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Albinism is a disorder caused by a recessive allele.About 1 out of every 75 people carry the albino allele.
• Let’s say a mom & dad with normal pigmentation have 4 children.
• 3 children have normal pigmentation, but 1 child is albino.
Where did the albino trait come from?
What is the genotype of the albino child?
What are the genotypes of the parents?
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The albino child’s genotype must be aa.
• Mom & dad are both heterozygous, Aa.
• The parents have normal pigmentation, but they are still carriers of the albino allele.
What are the phenotypic ratios?
What are the genotypic ratios?
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Have you noticed?
• When both parents are heterozygous, there’s a phenotypic ratio of 3:1 in the offspring.
75% green 25% yellow
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3:1 phenotypic ratio
75% purple25% white
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Monohybrid Crosses
• So far, we have only followed the inheritance of 1 trait at a time.
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Dihybrid Crosses
• We can also follow the inheritance of 2 traits at a time.
In this punnett square, we’re following 2 traits – seed
color & seed shape.
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Remember – The Law of Independent Assortment
• The presence of 1 specific allele for a trait has no impact on the presence of a specific allele for the other trait.
You can have blue eyes, no matter what color hair you have.
You can have red hair, no matter what color
eyes you have.
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How to Setup a Dihybrid Cross
• You’re following the inheritance of 2 traits at a time.– Tail Length: S = short, s = long– Fur Color: B = brown, b = white
• Your punnett square should have 16 squares.
• The possible gametes (egg & sperm) of the parents go on the outside of the punnett square.
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Easy Method for Finding Gametes – FOILfirst, outside, inside, last
• Dad’s genotype is RrYy.
What are the possible sperm cells that can be produced?
• First – RY• Outside – Ry• Inside – rY • Last – rY
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Finding the Possible Gametes
• Mom is heterozygous for both traits, SsBb. Her egg cells could contain:– SB, short tail & brown fur– Sb, short tail & white fur– sB, long tail & brown fur– sb, long tail & white fur
• Dad has the same genotype, so the results are the same for his sperm cells.
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In guinea pig fur… B = black, b = brown, S = short, s = long
Let’s say both parents are heterozygous for both traits.
BbSs x BbSs
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If both parents are heterozygous for both
traits, you will see a 9:3:3:1 phenotypic ratio.
BbSs x BbSs
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There are 4 possible sperm & egg gametes: YR, Yr, yR, yr
This means there are 16 possible ways the gametes
can combine to make offspring.
These combinations produce 4 distinct phenotypes in a 9:3:3:1 ratio, when both
parents are heterozygous for both traits.
Both parents are heterozygous for both traits, YyRr.
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