31/01/2013 § Leave a comment
Meet Gregor Mendel, the father of genetics.
He was perhaps the first scientist that experimented with the factors of organisms that could be passed on from generation to generation. Mendel worked with pea plants and when he crossed a variety with one trait and a variety of the opposite trait (i.e. wrinkled vs smooth), he found that all of the offspring would have the same trait as one of the parents. For example, all the peas in the F1 generation (the first generation) would be smooth. When Mendel let the first generation fertilize and pollenate itself, he found that the F2 generation showed both traits from the original parents, so both smooth and wrinkled peas were shown. The traits are controlled by the different forms of a gene called the alleles.
- homozygous, as implied in its name, are identical alleles in a gene; all gametes of a homozygote have the same allele
- heterozygous, quite the opposite of homozygous, are two different alleles in a gene; where half of the gametes of a heterozygote have one allele and the other half has the other allele
- dominant (alleles) are expressed when present in a heterozygote or a homozygote (so whether there is one dominant allele or two dominant alleles present)
- recessive (alleles) are expressed only in homozygotes (where there are two of the alleles present)
- genotype is all the alleles an organism possesses
- phenotype the characteristics of an organism
- segregation occurs in F1, when two alleles separate
- Punnett grid is what is used to show all possible outcomes after a cross (named after the scientist, Punnett, who first used the grid)
- Define the terms gene and allele and explain how they differ. 4 marks
- Describe the consequences of a base substitution mutation with regards to sickle cell anemia. 7 marks
- Outline the formation of chiasmata during crossing over. 5 marks
- Explain how an error in meiosis can lead to Down syndrome. 8 marks
- Karyotyping involves arranging the chromosomes of an individual into pairs. Describe one application of this process, including the way in which the chromosomes are obtained. 5 marks
- Compare the processes of mitosis and meiosis. 6 marks
- Outline one example of inheritance involving multiple alleles. 5 marks
- Describe the inheritance of ABO blood groups including an example of the possible outcomes of a homozygous blood group A mother having a child with a blood group O father. 5 marks
- Outline sex linkage. 5 marks
- Explain, using a named example, why many sex-linked diseases occur more frequently in men than women. 9 marks
DATA BASED QUESTIONS
Page 145, coat colour in the house mouse
In the early years of the 20th century, many crossing experiments were done in a similar way to those of Mendel. The French geneticist Lucien Cuénot used the house mouse, Mus musculus, to see whether the principles that Mendel had discovered also operated in animals. He crossed normal grey-coloured mice with albino mice. The hybrid mice that were produced were all grey. These grey hybrids were crossed together and produced 198 grey and 72 albino offspring.
1. Calculate the ratio between grey and albino offspring, showing your working.
198 + 72 = 270 –> 198 ÷ 270 = ~0.73 –> 72 ÷ 270 = ~0.26 –> 198:72 = ~75%:25% = ¾:¼ = 3:1
2. Deduce the colour of coat that is due to a recessive allele, with two reasons for your answer.
The albino offsprings carry the recessive allele because they only show up in the F2 generation, and there is a smaller amount of them among the 270 individual organisms. The grey mice appear more because ¾ of the possibilities contain the dominant allele that codes for the colour grey.
3. Choose suitable symbols for the alleles for grey and albino coat and list the possible genotypes of mice using your symbols, together with the phenotype for each genotype.
Grey = C, albino = c.
- CC = grey
- Cc = grey
- cC = grey
- cc = albino
4. Using the headings shown to the right, explain how the observed ratio of grey and albino mice was produced.
Whatever genotype the original parents in the experiment had, they passed on those genes to the F1 generation. In this generation, the parental phenotypes were all grey – all expressed the dominant allele. The parental genotypes were the homozygous version (CC) and the heterozygous versions (Cc & cC). The alleles of the gametes then produce the next generation of offspring, expressing more of the recessive allele when the genotype cc occurs. The 3:1 ratio is shown because after the F2 generation, the grey mice have three out of four chances among the mix of alleles, and the albino mice only have one chance to be expressed. The options above show what the four possible sequence of alleles are (#3).
5. Suggest how one gene can determine whether the mice had grey fur and black eyes or white fur and red eyes.
One gene can determine whether the mice had grey fur and black eyes or white fur and red eyes because a gene holds multiple alleles that can code for multiple things. Albino animals, for example, almost always have the two traits white fur (hair/skin/etc.) and red eyes – the two traits always go hand in hand. They must then be shared on the same gene but at a different locus to be expressed at the same time.