Lecture 18 Basics: Genes and Alleles
18.1 Basic vocabulary
Gene:
Allele:
Homologous chromosomes:
Homozygous vs heterozygous
Genotype:
Phenotype:
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
following.
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?
________________
f.
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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