Genetics and Adaptation Higher Biology Unit 2
Variation • Genes and Inheritance Shortly before a cell divides, the appearance of its nucleus changes. Long threads become visible in the nucleus, these are the chromosomes.
The number of chromosomes, and their size and shape varies between species.
When viewed under the electron microscope, each chromosome is seen to consist of many dark bands. These are the genes, each of which is responsible for controlling one characteristic in an organism.
Cell Division There are two types: • Mitosis (normal cell division in growing organisms) • Meiosis (takes place in gamete mother cells in the sex organs to produce gametes).
Mitosis This is simple cell division forming new cells (daughter cells) containing the same number of chromosomes as the mother cell.
In mitosis the number of chromosomes stays the same (46 in humans). This is called the diploid number.
3 1 5 4
Meiosis The genetic difference in gametes is the result of cell division in the sex cells called meiosis. During meiosis each diploid gamete mother cell undergoes two divisions to produce four haploid gametes.
The diploid number (2n) is the full chromosome number (complement) in normal cells. The haploid number (n) is half the diploid number. Only gametes have this number.
In a diploid cell, chromosomes can be sorted into pairs which look the same, and contain genes for the same characteristics. • These pairs are called homologous pairs. • Haploid gametes contain one member of each homologous pair.
How meiosis increases variation • Crossing over This takes place on the spindle during the first division of meiosis. Small pieces are exchanged between the chromosomes of a homologous pair.
Chromatid Centromere Exchanged pieces Chiasma (crossing over point)
2. Independent Assortment When homologous pairs of chromosomes line up at the equator of the spindle (during the first division of meiosis) the position of one pair is random in relation to any other pair.
X X X X X X X X X X X X X X X X X X
Genetics Genetics is the study of patterns of inheritance from one generation to the next.
Monohybrid cross Revision from Standard Grade/Int 2.
Dihybrid Cross This is a cross involving the inheritance of two characteristics. In pea plants the seeds (peas) can be either round or wrinkled, and either yellow or green. Round and Yellow are the dominant alleles.
Round = R Wrinkled = r Yellow = Y Green = y Cross the true-breeding round yellow with the true breeding wrinkled green:
Resulting phenotypes: Round and yellow = 9 Round and green = 3 Wrinkled and yellow = 3 Wrinkled and green = 1
Linked genes If two genes are on the same chromosome they are said to be linked. Linked genes are transmitted together.
e.g. In peas, the gene for plant height and seed colour are on the same chromosome (i.e. they are linked)
T = tall, t = short, Y = yellow, y = green Tall Yellow X Short Green TT YY tt yy TY ty All offspring will be TALL and YELLOW
Only 2 types of gamete possible If two F1 plants are crossed: TtYy x TtYy TY ty TY ty 3 Tall Yellow : 1 Short Green
In reality, in the above cross, if 400 seeds grew from the F2 the ratio might be: 292 : 7 : 6 : 95 Tall Yellow Tall Green Short Yellow Short Green Recombinants
The “Tall green” and “Short yellow” plants are possible because of crossing over during meiosis. This can “unlink” linked genes. The new forms are called recombinants.
Frequency of recombination Chiasmata can occur at any point along the length of homologous chromosomes. Genes that are further apart are more likely to be separated by crossing over than close genes. Recombinants gametes are therefore more likely to be formed.
B C A B C A b c a b c a Low frequency of recombination Higher frequency of recombination
The distance between a pair of linked genes is therefore indicated by the percentage number of F2 recombinants produced during a cross involving these genes.
This percentage is called the recombination frequency and is calculated as follows: number of F2 recombinants COV = x 100 total number of F2 offspring Recombination Frequency
In the example of the peas, the 400 F2 offspring: 292 : 7 : 6 : 95 Tall Yellow Tall Green Short Yellow Short Green Recombinants 13 Recombination = x 100 = 3.25 % Frequency 400
Chromosome maps Chromosome maps are used to show the position of genes on a chromosomes relative to one another. A large recombination frequency means that genes are far apart; a small frequency means that they are close together.
For example: Crosses involving 4 linked genes (ABDE) gave the following Recombination frequencies:
The positions of the genes on the chromosome are therefore as follows: E B D A 2 6 6 12
Sex Determination Diploid human body cells have 46 chromosomes. These are made up of 22 normal homologous pairs (called autosomes) and one pair of sex chromosomes.
The sex chromosomes in woman are two similar “X” chromosomes. In men there is one “X” chromosome and a smaller “Y” chromosome. XX XY
The “X” chromosomes carry many genes (unrelated to sex). The “Y” carries no genes.