Checkpoint 4 - IQ4: Genetic Variation and Comparison | HSC Biology Year 12 Module 5

What's Covered

L13

Sources of Genetic Variation

Crossing over and independent assortment
Random fertilisation
Mutation as a source of new alleles
Variation from reshuffling versus new DNA change

L14

Mendelian Patterns

Autosomal dominant and recessive inheritance
X-linked inheritance
Punnett square probability
Pedigree clues and interpretation

L15

Non-Mendelian Patterns

Co-dominance
Incomplete dominance
Multiple alleles
ABO blood group reasoning

L16

Frequency Data and SNPs

Trait frequency tables
Trends, relationships and limitations
Sample size and bias
SNPs as markers and their limits

Core Reasoning

Genetic variation comes from meiosis, fertilisation and mutation; inheritance models help predict genotype outcomes; data comparisons require careful interpretation.

Common Trap

Crossing over reshuffles existing alleles. Mutation creates new alleles. These are not the same process.

Data Warning

One marker or one sample can suggest a pattern, but strong conclusions need larger, representative evidence.

Section A - Multiple Choice (10 questions)

Question 1

Which process creates new allele combinations without creating new alleles?

A Mutation

B DNA replication error only

C Crossing over during meiosis

D Translation at the ribosome

Question 2

Why does random fertilisation increase variation?

A Different sperm and egg combinations can fuse, producing many possible genotypes.

B It changes every gene by mutation.

C It guarantees each family follows an exact ratio of outcomes.

D It converts recessive alleles into dominant alleles.

Question 3

Which statement about a Punnett square is correct?

A It guarantees the exact order of births in a family.

B It proves which mutation occurred in the parents.

C It shows phenotype only and cannot show genotype.

D It models possible offspring genotypes and their probabilities.

Question 4

Which clue most strongly supports an X-linked recessive pattern?

A The trait appears in both sexes.

B There is no father-to-son transmission of the affected allele.

C Unaffected parents can have an affected child.

D The trait can skip generations.

Question 5

Which genotype gives blood group AB?

A IAIB

B ii

C IAi

D IBi

Question 6

A 1:2:1 phenotype ratio from a heterozygous cross most strongly suggests what?

A The gene must be Y-linked.

B The population sample was biased.

C Every allele is recessive.

D The heterozygote has a distinct phenotype, such as in incomplete dominance or co-dominance.

Question 7

What is the best description of multiple alleles?

A One individual carries more than two alleles for one gene at the same time.

B More than two allele forms of a gene exist in the population.

C Every allele is expressed equally in every heterozygote.

D A gene is found on many different chromosomes.

Question 8

Population A shows a trait frequency of 68%, while Population B shows 39%. Which interpretation is best?

A The trait occurs in every organism in Population A.

B Population B cannot possess the trait genetically.

C The trait is more common in Population A in the sampled data.

D The two populations must be different species.

Question 9

What is a SNP?

A A one-base difference at a specific DNA position

B A full chromosome inversion in every cell

C A protein that controls meiosis

D A type of gamete

Question 10

Why is one SNP alone limited as evidence?

A Because SNPs cannot differ between individuals

B Because one SNP automatically determines the phenotype of every trait

C Because one SNP removes the need for any sample data

D Because one marker does not represent the whole genome or all relatedness patterns

Section B - Short Answer

Question 11

Explain the difference between variation caused by crossing over and variation caused by mutation.

4 marks

Crossing over reshuffles existing alleles between homologous chromosomes during meiosis, creating new combinations of alleles. Mutation changes the DNA sequence itself and can create a new allele. The key difference is that crossing over reorganises what is already present, while mutation produces new DNA variation.

Question 12

Two pink flowers are crossed in a species showing incomplete dominance. Red flowers are CRCR, white flowers are CWCW, and pink flowers are CRCW. Determine the genotype ratio and phenotype ratio.

4 marks

The cross is CRCW x CRCW. Each parent produces gametes CR and CW. The offspring genotypes are CRCR, CRCW, CRCW and CWCW. Genotype ratio = 1 CRCR : 2 CRCW : 1 CWCW. Phenotype ratio = 1 red : 2 pink : 1 white.

Question 13

A study compares one SNP across two populations and finds different frequencies. Explain what conclusion can reasonably be drawn and give one limitation of the evidence.

5 marks

A reasonable conclusion is that the sampled populations differ at that SNP marker, so the data suggests some genetic difference in the sampled groups. However, one SNP is only one marker and does not describe the whole genome. The conclusion is also limited by factors such as sample size, whether the sample is representative, and possible sampling bias.

Question 14

A pea plant heterozygous for seed shape and seed colour (RrYy) is crossed with an identical RrYy plant. The two genes are on different chromosomes and assort independently. State the four gamete types each parent can produce, give the expected phenotypic ratio of the offspring, and explain why this ratio arises.

4 marks

Each RrYy parent produces four gamete types in equal proportion: RY, Ry, rY and ry. Combining these in a 4x4 Punnett square gives an expected phenotypic ratio of 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green, that is 9:3:3:1. This ratio arises because the two genes assort independently during meiosis (law of independent assortment), so each 3:1 monohybrid ratio combines, and it depends on the genes being unlinked.

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