What is the difference between homozygous and heterozygous?

Homozygous has two identical alleles. Heterozygous has two different alleles.

Both alleles are expressed in the phenotype of a heterozygote.

What are multiple alleles?

Three or more alleles of a gene in a population, although each person has only two.

What does it show if two unaffected parents have an affected child?

The condition is recessive and both parents are heterozygous carriers.

What is autosomal linkage?

Two genes on the same autosome, so their alleles tend to be inherited together.

Why do recombinants appear in a linkage test cross?

Crossing over between non-sister chromatids in prophase I makes new combinations of alleles.

Why is haemophilia more common in males?

The allele is on the X chromosome and males have only one X, so one recessive allele gives the condition.

Write the formula for chi-squared.

\chi^{2} = \sum \frac{(O-E)^{2}}{E}

What do you conclude if \chi^{2} is greater than the critical value at p = 0.05?

Reject the null hypothesis: the difference between observed and expected results is significant.

Genetic crosses and inheritanceEdexcel A-Level Biology B: Flashcards

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Define genotype and phenotype.

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Define genotype and phenotype.: Genotype is the combination of alleles an organism has. Phenotype is the observable characteristic produced by the genotype and the environment.
What is the difference between homozygous and heterozygous?: Homozygous has two identical alleles. Heterozygous has two different alleles.
What is codominance?: Both alleles are expressed in the phenotype of a heterozygote.
What are multiple alleles?: Three or more alleles of a gene in a population, although each person has only two.
What does it show if two unaffected parents have an affected child?: The condition is recessive and both parents are heterozygous carriers.
What phenotype ratio comes from RrYy $\times$ RrYy (unlinked genes)?: 9:3:3:1.
What phenotype ratio comes from RrYy $\times$ rryy (unlinked genes)?: 1:1:1:1.
What is autosomal linkage?: Two genes on the same autosome, so their alleles tend to be inherited together.
Why do recombinants appear in a linkage test cross?: Crossing over between non-sister chromatids in prophase I makes new combinations of alleles.
In a linkage test cross, are parental types or recombinants more common?: Parental types.
Why is haemophilia more common in males?: The allele is on the X chromosome and males have only one X, so one recessive allele gives the condition.
Write the formula for chi-squared.: $\chi^{2} = \sum \frac{(O-E)^{2}}{E}$
How do you work out degrees of freedom for chi-squared?: Number of classes minus one.
What do you conclude if $\chi^{2}$ is greater than the critical value at $p = 0.05$ ?: Reject the null hypothesis: the difference between observed and expected results is significant.

Exam questions on Genetic crosses and inheritance

A mother with blood group A and a father with blood group B have a first child with blood group O. The ABO blood group gene has three alleles: $I^{A}$ and $I^{B}$ , which are codominant, and $I^{O}$ , which is recessive to both.
Explain how two parents with blood groups A and B can have a child with blood group O.2 marks
In pea plants, round seeds (R) are dominant to wrinkled seeds (r) and yellow seeds (Y) are dominant to green seeds (y). The two genes are on different chromosomes. A plant heterozygous for both genes has the genotype RrYy.
State the genotypes of the four types of gamete made by a plant of genotype RrYy and explain why they are produced in equal numbers.2 marks
Haemophilia A is caused by a recessive allele, $X^{h}$ , on the X chromosome. The dominant allele, $X^{H}$ , gives normal blood clotting. A woman who is a carrier of haemophilia ( $X^{H}X^{h}$ ) has a child with a man who does not have haemophilia.
Construct a genetic cross to show the genotypes of the possible children and state the probability that the next child will have haemophilia.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).