4. Dominance The gene pairs control one characteristic, but they do not always control it in the same way. Of the gene pair which help determine coat colour in mice, one might try to produce black fur and its partner might try to produce brown fur. The gene for black fur is dominant to the gene for brown fur. 3
5.
6.
7.
8.
9. meiosis meiosis fertilization All offspring will be black (Bb) sperm mother cell ovum mother cell zygote B B B B b b b b B b
10. -The offspring from this cross are called the F 1 (First Filial) generation -They are all black because the allele for black coat colour is dominant to the allele for brown coat colour -These Bb mice are called heterozygotes . Because the B and b alleles have different effects; producing either black or brown coat colour The mice are heterozygous for coat colour -The BB mice are called homozygotes because the two alleles produce the same effect. Both alleles produce black coats. -The bb mice are also homozygous for coat colour. Both alleles produce a brown coat colour -The next slide shows what happens when the two heterozygotes are mated and produce young 8
11. F 2 BB Bb Bb bb sperm mother cell ovum mother cell meiosis Possible combinations Fertilization sperms ova zygotes 9 B b B b B b B b B B B b B b b b
12.
13.
14.
15. These tobacco seedlings are the F 2 generation from a cross Between heterozygous ( Cc ) parents. C is the gene for chlorophyll. cc plants can make no chlorophyll. There are 75 green seedlings present. What is the ratio of green to white seedlings? What ratio would you expect? 13
16. There are 21 white seedlings. This is a ratio of 75:21 or 3.57:1 Is 3.57:1 near enough to 3:1 ?* 1 CC 2 Cc and 1 cc , a ratio of 3 green to 1 white seedling You would expect the cross to produce 72 green to 24 white seedlings (3:1) c C c C cc 14 CC Cc Cc
17. Sex chromosomes In most populations of animals there are approximately equal numbers of males and females. This is the result of a pair of chromosomes; the sex chromosomes called the X and Y chromosomes. The X and Y chromosomes are a homologous pair but in many animals the Y chromosome is smaller than the X. Females have two X chromosomes in their cells. Males have one X and one Y in their cells. At meiosis, the sex chromosomes are separated so the the gametes receive only one: either an X or a Y . 15
18. Sex ratio sperm mother cell ovum mother cell meiosis fertilization female female male male 16 X Y X X X Y X X X X X X X Y X Y
19. Single gene effects - Very few human characteristics are controlled by a single gene - Characteristics such as height or skin colour are controlled by several genes acting together - Those characteristics which are controlled by a single gene are usually responsible for inherited defects ( see slide 19 ) 17
20.
21. Genetic defects Cystic fibrosis (recessive) Glands of the alimentary canal produce a thick mucus which affects breathing, digestion and susceptibility to chest infection Achondroplastic dwarfism (dominant)The head and trunk grow normally but the limbs remain short Albinism (recessive) Albinos cannot to produce pigment in their skin, hair or iris Polydactyly (dominant*) an extra digit may be produced on the hands or feet Sickle cell anaemia (recessive)The red blood cells become distorted if the oxygen concentration falls. They tend to block small blood vessels in the joints 19
22. Genetic counselling If the genotypes of the parents are known, it is possible to calculate the probability of their having an affected child (i.e. one with the defect) For example if a male achondroplastic dwarf marries a normal woman, what are their chances of having an affected child? The father’s genotype must be Dd . ( DD is not viable) The mother must be dd since she is not a dwarf There is a 50% probability of their having an affected child What are the probabilities if both parents are affected? (Genetic defects) 20 D d d d Dd Dd dd dd
23. Cystic fibrosis If two normal parents have an affected child, they must both be heterozygous ( Nn ) for the recessive allele n NN Nn Nn nn A nn parent would have cystic fibrosis A NN parent would produce only normal children Since the parents are now known to be heterozygous it can be predicted that their next child has a I in 4 chance of inheriting the disease This chance applies to all subsequent children* (recessive) 21 N n N n
24.
25. Carriers Heterozygous recessive individuals do not usually exhibit any disease symptoms but because their offspring may inherit the disease, the heterozygotes are called ‘ carriers ’ Hb A Hb A Hb A Hb A Hb A Hb S Hb A Hb S Hb S Hb S Hb S Hb S carriers Similarly, individuals with the genotype Nn are carriers for cystic fibrosis 23
26.
27. If one of the parents is homozygous for a dominant allele, all the children will be affected If one parent is heterozygous for a dominant allele and the other is homozygous recessive, there is a chance that half their children will be affected If both parents are heterozygous for a recessive allele, there is a chance that one in four of their children will be affected 25 AA Aa aa Aa Aa
28. grandparents parents children cystic fibrosis What can you deduce about the genotypes of the grandparents from this family tree? marriage marriage 26
29. Cystic fibrosis is caused by a recessive gene An affected person must therefore have the genotype nn Since neither of the grandparents is affected, they must be either NN or Nn genotypes If they were both NN , none of their children or grandchildren could be affected If one was Nn and the other NN , then there is a chance that 50% of their children could be carriers Nn If one of the carriers marries another carrier, there is a 1 in 4 chance of their having an affected child The genotypes of the grand parents must be either both Nn or one NN and the other Nn 27
30. If both parents have the Dd genotype there is a 75% chance of their having affected children, but the DD individual is unlikely to survive D D DD Dd Dd dd d d 28
31. Question 1 Which of the following are heterozygous genotypes? (a) Aa (b) bb (c) nn (d) Bb 29
32. Question 2 Which of these genes are alleles? (a) A and A (b) A and B (c) B and C (d) B and b chromosomes 30 A B C A b c
33. Question 3 Which of the following processes separates homologous chromosomes ? (a) mitosis (b) cell division (c) meiosis (d) fertilization 31
34. Question 4 Which of the following terms correctly describes the genotype bb ? (a) homozygous dominant (b) heterozygous dominant (c) homozygous recessive (d) heterozygous recessive 32
35.
36.
37. Question 7 What is the expected ratio of offspring from a black rabbit Bb and a white rabbit bb ? (c) 50% white; 50% black (a) 3 black: 1 white (b) 1 black: 3 white (d) all black 35
38. Question 8 Which of these Punnett squares correctly represents a cross between two heterozygous individuals ? AA AA AA aa aa a Aa Aa aa Aa aa aa Aa AA Aa Aa Aa (a) (b) (c) (d) 36 A a A a A a A a A a A a a A a
39. Question 9 A married couple has a family of 6 boys. What are the chances that the next child will be a girl ? (d) 1:1 (a) 6:1 (b) 1:6 (c) 3:1 37
40. Question 10 Which of the following is a ‘carrier’ genotype for a disease caused by a recessive gene ? (a) nn (b) NN (c) Nn 38
The slide shows tobacco seedlings some of which have no chlorophyll, See slide No.13
* After Dr. R.C. Punnett, Professor of Genetics in Cambridge in the first half of the 20 th century.
The 3:1 ratio is sometimes called a Mendelian ratio after Gregor Mendel (1822-84), an Austrian Monk with an interest in plant breeding and mathematics. He made experimental crosses, particularly of pea plants, to see how characteristics were inherited. He realised that although certain characteristics seemed to disappear in the first generation, they could reappear in the second generation, and so he developed the idea of dominant and recessive characteristics with the recessive characteristics being suppressed in the first generation.. His second generation crosses, involving single characteristics, produced ratios approximating to 3;1. From these results he drew accurate conclusions about the mechanism of inheritance although he knew nothing about chromosomes and genes at this time.
*There are 75 green seedlings + 21 white seedlings. This is the ‘observed result’. If the 3:1 ratio is operating, the ‘expected result’ would be 72 green and 24 white seedlings. A statistical test on these figures tells us that the difference between the observed and expected results is sufficiently small to be ignored and the 75:21 ratio is near enough to 3:1 to be confident that this is the outcome of a genetic process.
This shows that there is a 50% chance of a boy or a girl baby. Because this ratio depends on the chance meeting of X and Y gametes and human families are small, the 1:1 ratio is rarely seen, even in large families. A family with 6 boys may hope for a girl next time but there is still only a 50% chance of getting a girl. In the population as a whole, the 1:1 ratio holds good.
*Even though the allele is dominant, it may not always be fully expressed. X-rays will reveal an extra metacarpal but this may not form a separate digit so the hand looks normal, In some cases only one of the hands shows the extra digit.
The incidence of achondroplasia is about 1 in 30000. These cases arise as a result of a mutation; the affected children are born to normal parents. A mutation is a spontaneous change in a gene or a chromosome. Mutations are mostly harmful.
Even if the parents have five normal children, there is still a 1 in 4 chance of the next child being affected. If the heterozygotes could be detected before they had children, they could be counselled about the likelihood of having affected children. Families at risk (I.e. cystic fibrosis in a relative) may be offered DNA testing to look for one or more of the genes which cause the disease.
![Terms you should know ,[object Object],[object Object],[object Object],[object Object],[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)