2 Diploid organisms contain two copies of each gene in each of their cells. In sexual reproduction, gametes are formed containing one copy of each gene. Each off spring receives two copies of each gene, one from each of its parents.
3 The cells produced by meiosis are genetically different from each other and from their parent
cell. This results from independent assortment of the chromosomes as the bivalents line up on the
equator during metaphase I, and also from crossing over between the chromatids of homologous
chromosomes during prophase I.
4 Genetic variation also results from random fertilisation, as gametes containing diff erent varieties
of genes fuse together to form a zygote.
5 An organism’s genetic constitution is its genotype. The observable expression of its genes is its phenotype.
6 Different varieties of a gene are called alleles. Alleles may show dominance, codominance or recessiveness. An organism possessing two identical alleles of a gene is homozygous; an organism possessing two different alleles of a gene is heterozygous. If a gene has several diff erent alleles, such as the gene for human blood groups, these are known as multiple alleles.
7 The position of a gene on a particular chromosome is
its locus.
8 A gene found on the X chromosome but not on the Y chromosome is known as a sex-linked gene.
9 The genotype of an organism showing dominant characteristics can be determined by looking at the off spring produced when it is crossed with an organism showing recessive characteristics. This is called a test cross.
10 Monohybrid crosses consider the inheritance of one gene. Dihybrid crosses consider the inheritance of two diff erent genes.
11 The χ2 test can be used to find out whether any diff erences between expected results and observed results of a genetic cross are due to chance, or whether the difference is significant.
12 The genotype of an organism gives it the potential to show a particular characteristic. In many cases, the degree to which this characteristic is shown is also affected by the organism’s environment.
13 Mutation can be defined as an unpredictable change in the base sequence in a DNA molecule (gene mutation) or in the structure or number of chromosomes (chromosome mutation). New alleles arise by gene mutation. Gene mutations include base substitutions, deletions or additions. The HbS (sickle cell) allele arose by base substitution. Such mutations may affect the organism’s phenotype.
8 A gene found on the X chromosome but not on the Y chromosome is known as a sex-linked gene.
9 The genotype of an organism showing dominant characteristics can be determined by looking at the off spring produced when it is crossed with an organism showing recessive characteristics. This is called a test cross.
10 Monohybrid crosses consider the inheritance of one gene. Dihybrid crosses consider the inheritance of two diff erent genes.
11 The χ2 test can be used to find out whether any diff erences between expected results and observed results of a genetic cross are due to chance, or whether the difference is significant.
12 The genotype of an organism gives it the potential to show a particular characteristic. In many cases, the degree to which this characteristic is shown is also affected by the organism’s environment.
13 Mutation can be defined as an unpredictable change in the base sequence in a DNA molecule (gene mutation) or in the structure or number of chromosomes (chromosome mutation). New alleles arise by gene mutation. Gene mutations include base substitutions, deletions or additions. The HbS (sickle cell) allele arose by base substitution. Such mutations may affect the organism’s phenotype.
1. End-of-chapter questions
1. A cell in the process of meiosis was seen to have a spindle with sister chromatids being drawn towards opposite poles of the cell. In what stage of meiosis was the cell? .
A anaphase I
B anaphase II
C metaphase I
D metaphase II
2 All the offspring of a cross between pure-bred red-flowered and pure-bred white-flowered snapdragons were pink.
Two of these pink-flowered plants were interbred. What proportion of the offspring were pink?
A 25%
B 33%
C 50%
D 100%
3 A man has haemophilia. Which statement correctly describes the inheritance of the gene causing his condition?
A He inherited the recessive allele from his mother.
B He inherited the dominant allele from his father.
C He can pass the recessive allele to a son.
D He can pass the dominant allele to a daughter.
4 The diploid (2n) chromosome number of Drosophila is 8. Copy and complete the table to show the different outcome of mitotic and meiotic division of a Drosophila cell.
5 Copy and complete the table to compare meiosis with mitosis.
6 a Describe the essential difference between meiosis I and meiosis II.
b State the similarity between meiosis II and mitosis.
7 There is no crossing over during meiosis in male Drosophila. Assuming that no mutation occurs, the only source of genetic variation is independent assortment. Given that the diploid (2n) chromosome number is 8, calculate the number of genetically different spermatozoa that can be produced.
8 Distinguish between the following pairs of terms.
a genotype and phenotype
b homozygous and heterozygous
9 In sweet-pea plants, the gene A/a controls flower colour. The dominant allele gives purple flowers and the recessive allele red flowers.
A second gene, B/b, controls the shape of the pollen grains. The dominant allele gives elongated grains and the recessive allele spherical grains.
A plant with the genotype AaBb was test-crossed by interbreeding it with a plant with red flowers and spherical pollen grains.
Copy and complete the table to show that the expected ratio of phenotypes of the offspring of this cross. The gametes from one parent are already in the table.
[5]
10 a The fruit fly, Drosophila melanogaster, feeds on sugars found in damaged fruits. A fly with normal features is called a wild type. It has a striped body and its wings are longer than its abdomen. There are mutant variations such as an ebony-coloured body or vestigial wings. These three types of fly are shown in the figure.
Wild-type features are coded for by dominant alleles: A for wild-type body and B for wild-type wings. Explain what is meant by the terms allele and dominant. [2]
b Two wild-type fruit flies were crossed. Each had alleles A and B and carried alleles for ebony body and vestigial wings.
Draw a genetic diagram to show the possible offspring of this cross. [6]
c When the two heterozygote flies in b were crossed, 384 eggs hatched and developed into adult flies.
A chi-squared (X2) test was carried out to test the significance of the differences between observed and expected results:
i Copy and complete the table.
ii Calculate the value for X2
As four classes of data were counted, the number of degrees of freedom was 4 - 1 = 3. The table gives values of X2 where there are three degrees of freedom.
iii Using your value for X2 and the table above, explain whether or not the observed results were significantly different from the expected results. [2]
[Cambridge International AS and A Level Biology 9700/41, Question 7, October/November2009]
b Two wild-type fruit flies were crossed. Each had alleles A and B and carried alleles for ebony body and vestigial wings.
Draw a genetic diagram to show the possible offspring of this cross. [6]
c When the two heterozygote flies in b were crossed, 384 eggs hatched and developed into adult flies.
A chi-squared (X2) test was carried out to test the significance of the differences between observed and expected results:
i Copy and complete the table.
[1]
As four classes of data were counted, the number of degrees of freedom was 4 - 1 = 3. The table gives values of X2 where there are three degrees of freedom.
[Total: 14]
[Cambridge International AS and A Level Biology 9700/41, Question 7, October/November2009]
2. End-of-chapter answers
1 B
2 C
3 A
Exam-style questions
2 C
3 A
6 a meiosis I: separates homologous chromosomes;
meiosis II: separates sister chromatids;
b both separate sister chromatids;
7 (2n , where n = 4) 2 × 2 × 2 × 2 = 16
8 a genotype: the genetic constitution of an organism
with respect to a gene or genes;
phenotype: the physical, detectable expression of
the particular alleles of a gene or genes present in
an individual;
b homozygous: describes a diploid organism that
has the same allele of a gene at the gene’s locus on
both copies of the homologous chromosome;
heterozygous: describes a diploid organism that
has diff erent alleles of a gene at the gene’s locus on
the homologous chromosomes;
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