Flashcards - Topic 7.1 Inheritance - AQA Biology A-level
(33 cards)
Define genotype.
The genetic constitution of an organism.
Define phenotype.
The expression of an organism’s genetic constitution, combined with its interaction with the environment.
What is an allele?
Different forms of a particular gene, found at the same locus (position) on a chromosome. A single gene could have many alleles.
How many alleles per gene do diploid organisms carry?
Two.
What is meant by a dominant allele?
An allele whose characteristic will always appear in the phenotype, whether one or two are present.
What is meant by a recessive allele?
An allele whose characteristic only appears in the phenotype if no dominant allele is present, meaning two must be present.
What is meant by codominant alleles?
Two dominant alleles that both contribute to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together.
What is meant by homozygous and heterozygous?
Homozygous= both alleles are dominant, or both alleles are recessive. Heterozygous= one allele is dominant, the other is recessive.
Define monohybrid inheritance.
Where one phenotypic characteristic is controlled by a single gene.
Draw a Punnett square to show a monohybrid cross with parental genotypes of GG and gg.
G
G
g
Gg
Gg
g
Gg
Gg
What % of these offspring will express the characteristic determined by allele G?
100%; all potential offspring genotypes are Gg meaning none of them will express the recessive allele.
Draw a Punnett square to show a monohybrid cross with parental genotypes of Gg and Gg.
G
g
G
GG
Gg
g
Gg
gg
What % of these offspring should express the characteristic determined by allele G?
75%; only one of the combinations results in expression of the recessive allele.
Draw an example of a genetic diagram.
Parental phenotypes Brown eyes Blue eyes
Parental genotypes Bb bb
Gametes
B b b b
Offspring genotypes Bb, Bb, bb, bb
(draw a punnet square)
Offspring phenotypes 2:2 brown eyes:blue eyes
Define dihybrid inheritance.
Where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time.
Draw a Punnett square to show a dihybrid cross with parental genotypes of RrGg and RrGg.
RG
Rg
rG
rg
RG
RRGG
RRGg
RrGG
RrGg
Rg
RRGg
RRgg
RrGg
Rrgg
rG
RrGG
RrGg
rrGG
rrGg
rg
RrGg
Rrgg
rrGg
rrgg
How many of these offspring should have the same phenotype as their parents?
9 (out of 16)
6 will match their parents on one characteristic, but differ on the other.
1 will differ on both characteristics.
What is meant by sex-linkage?
Where an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual.
Why are males more likely to express a recessive sex-linked allele?
Most sex-linked alleles are located on the X chromosome. Therefore males only get one copy of the allele, so will express this characteristic even if it’s recessive. Since females get two alleles, this is less likely.
Which parent do males inherit sex-linked characteristics from?
Their mother, since the Y chromosome can only come from their father. Therefore if the mother is heterozygous for sex-linked alleles, she is a carrier and may pass on the trait on.
Draw a Punnett square to show a sex-linked cross with parental genotypes of XHXh and XHY.
XH
Xh
XH
XHXH
XHXh
Y
XHY
XhY
Describe the four possible phenotypes of these offspring.
Normal female, carrier female, normal male, affected male.
What is meant by autosomal linkage?
Where two or more genes are located on the same (non-sex) chromosome. In this case, only one homologous pair is needed for all four alleles to be present. For genes that aren’t linked, two homologous pairs are needed.
Draw a Punnett square to show a multiple alleles cross with parental genotypes of IAIO and IBIO.
IA
IO
IB
IAIB
IBIO
IO
IAIO
IOIO
