1. What is an "allele" ?
Answer
A variant form of a gene.
2. An organism can be described as homozygous or heterozygous for a particular gene. Explain what these two terms mean.
Answer
Homozygous if they have two of the same allele. Heterozygous if they have two different alleles.
3. What is a sex-linked gene ? Why are sex-linked conditions more common in males ?
Answer
Gene is located on the X chromosome. Males have only one X chromosome while females have two.
4. Red-green colour blindness is caused by a recessive allele for a gene located on the X chromosome. There are two alleles for this gene, XB for normal vision and Xb for red-green colour blindness. The figure below shows the results the family tree of one family. Identify evidence in the diagram that shows that the red-green colour blindness allele is recessive.
Answer
1 and 2 are both not colourblind, but have an offspring 3 who is colourblind. Individual 2 must carry allele for colour blindness in order to have offspring 3.
Individuals 6 and 7 have offspring that is colourblind. Individual 6 must carry allele for colour blindness in order to have offspring 8.
5. Identify the genotype of individual 6.
Answer
Heterozygous - XBXb
6. Scientists investigated the inheritance of two characteristics in sweet peas: flower colour and pollen grain shape. There are two alleles for each gene.
Flower colour is determined by the alleles B (blue) and b (red).
Pollen grain shape is determined by the alleles R (long) and r (short).
Two heterozygous plants were crossed. Give the expected phenotypic ratios be for these offspring. Use a genetic diagram.
Answer
Parent genotype: BbRr and BbRr
The expected phenotypic ratio is 9:3:3:1
7. Explain why the observed phenotypic ratio for these for these offspring may be different than the expected value.
Answer
The genes are linked, meaning they are located on the same chromosome. The alleles BR and br are inherited together, so the majority of gametes produced by both plants are either BR or br. Other allele combinations are only generated through crossing over. Since crossing over between these two gene loci is rare, fewer recombinant gametes are produced, which explains the lower numbers of blue short and red long offspring.
8. Scientists carried out a cross in pea plants, looking at two different characteristics of seeds.
Smooth seeds is determined by the R allele and wrinkled by r.
Yellow seed colour is determined by G and green colour by g.
Two smooth yellow pea plants were crossed and the phenotypic frequencies of the offspring were observed and shown in the table below. Identify the genotypes of the parent plants.
Answer
Both parents are RrGg.
9. Complete the table below. Round your figures to give whole numbers.
Answer
10. Scientists calculated that χ2 = 4.51. Use the table of critical values shown below to explain what this value of χ2 means.
Answer
The value 4.51 is less than the critical value of 7.81. Therefore, there is more than a 10% chance that the difference between the observed and expected results is due to chance. The difference is not statistically significant, and the null hypothesis is supported.
11. Give the null hypothesis that was used in this test.
Answer
There is a 9:3:3:1 ratio of phenotypes in these offspring.