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Vocabulary Review
 Allele
 Dominant vs. Recessive
 Heterozygous vs. Homozygous
 Phenotype vs. Genotype

Law of Segregation
 allele pairs separate or segregate during gamete
formation, and randomly unite at fertilization.

Monohybrid Crosses (Punnett
Squares)

 Genotype Ratio?
1AA:2Aa:1aa

 Phenotype Ratio?
3dominant:1recessive

Punnett Squares
 Things to consider:
 Which trait is dominant and which is recessive?
 Is each parent heterozygous or homozygous (dominant
or recessive)?
 What allele abbreviation would we use?
 Genotype and Phenotype ratios can be DIFFERENT!!!

Molly and Neil are planning to have
children. Molly has blue eyes and Neil has
brown eyes. What color eyes could their
children have?

**Neil’s mom had blue
eyes and his dad was
homozygous for brown
eyes**

 Now we know:
 Molly=bb
 Neil=Bb
 Genotype Ratio:
1Bb:1bb

 Phenotype Ratio:
1 brown eyes: 1 blue
eyes

In plants, tall height (T) is dominant to short (t). A homozygous
dominant plant is crossed with a heterozygous plant. What would
be the resulting phenotypic and genotypic ratios in the resulting
generation of plants?

Incomplete Dominance
 form of intermediate inheritance in which one allele for a specific
trait is not completely dominant over the other allele resulting in a
third phenotype that is a combination of the dominant and
recessive phenotypes

 Genotypic Ratio?
1:2:1
(WW:RW:RR)

 Phenotypic Ratio?
1:2:1
(white:pink:red)

Sex-linked Traits (Examples: red-
green color blindness, hemophilia)
 xBxB : normal female
 xBxb: carrier female
 xbxb :affected female
 xBy: normal male
 xby: affected male

Cross a male with hemophilia with a
normal woman whose father had
hemophilia.
Xh y
Genotypic ratio: 1xHXh
x HXh xHy :1xHy: 1Xh Xh : 1Xhy
xH
Phenotypic ratio: 1 normal
female: 1 hemophiliac female:
X h Xh Xhy 1 normal male: 1 hemophiliac
Xh male

Review Questions
 A male with hemophilia marries a normal woman whose
father had hemophilia. What are the chances of their sons
and daughters having hemophilia? Be sure to give the
phenotypic and genotypic ratios.
 A normal male marries a woman who is also normal but
whose father was colorblind. What are the chances of
their sons and daughters being color blind? Give the
phenotypic and genotypic ratios.

Blood Type: Multiple Alleles
 Blood types:
 A, B, AB, and O
 Alleles
 Codominance: A & B
 I A and IB (can be homozygous or heterozygous)
 Recessive: O
 i

A female with type AB blood is crossed with a
man who has type O blood. What are the
genotypic and phenotypic combinations their
children could have?
 Parents: I AIB x ii
IA IB
 Genotypic Ratio: 1 I A i: 1 I B i IA i IB i
 Phenotypic ratio: 1 type A: 1 type B i
IA i IB i
i

*Look over dihybrid
cross notes*

Pedigrees: chart of an individual's ancestors used in
human genetics to analyze Mendelian inheritance of
certain traits, especially of familial diseases.

Pedigrees

Female Male
Offspring

Pedigrees

Individuals can be shaded to track certain traits.

Female with specific trait Male with specific trait

Pedigree Key:

& = color blind

& = normal vision xBxb xby

What would be the
genotype of each of
xbxb xBy
the individuals?

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Genetics 137

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Genetics 137