Lecture 18 Basics: Genes and Alleles 18.1 Basic vocabulary Gene

Lecture 18 Basics: Genes and Alleles
18.1 Basic vocabulary
Homologous chromosomes:
Homozygous vs heterozygous
18.2 Dominance
As Mendel understood it:
Offspring are purple
Therefore, the “big P” purple allele is
dominant to the “little p” white allele.
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Dominance Explained:
Pigment that causes purple color: anthocyanin
2 Genes: transcription factor on one chromosome, enzyme gene on another chromosome
The “Big P” allele
The “Little p” allele
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18.3 Homozygotes and heterozygotes
Homozygous PP
Homozygous pp
Heterozygous Pp
Dominant allele:
Recessive allele:
18.4 Punnett Squares
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Punnet Square Practice:
Book question 14-6
A pea plant heterozygous for inflated pods (Ii) is crossed with a plant homozygous for
constricted pods. Draw a Punnett square for this cross. What percentage of the offspring will
have constricted pods?
Genotype is different from phenotype
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18.5 The Test cross: determines genotype in an organism showing the dominant trait
Given an unknown:
Solution: cross unknown against __________________________________________
Offspring if PP:
Offspring if Pp:
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Practice problems:
1. In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that
are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?
2. In guinea pigs, short hair is dominant to long hair (S,s). Use a Punnett square to determine the
a. One guinea pig has long hair, the other is a heterozygote.
What proportion of the offspring will have short hair?
b. A litter of guinea pigs all have short hair. One parent has short hair,
the other long hair. What is the genotype of the short haired guinea pig?
3. In humans, being a tongue-roller is dominant over non-roller (R, r). A man who is a non-roller marries a
woman who is a heterozygous for tongue-rolling.
a. What is the father’s phenotype?
b. What is the father’s genotype?
c. What are the possible gametes the father can give his offspring?
d. What are the possible gametes the mother can give her offspring?
e. What is the probability their child will be a tongue-roller?
They have a child who is a tongue-roller. If she marries a man
who is a non-roller, what is the probability their child will be a tongue-roller? ______________
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