1. Genetics The Science of Heredity and Variation

2. 3 4 3 4 2 2 5 5 1 1 8 9 10 8 9 10 7 11 12 7 11 12 6 6 14 14 15 16 17 18 15 16 17 18 13 13 23 23 19 19 20 22 20 22 21 21
Mum 3 4
Dad 2 5 1 8 9 10 7 11 12 6 14 15 16 17 18 13
Child 23 19 20 22 21

3. 1 2 3 4 1 2 3 4
Cell in the ovary
Cell in the testes 1 2 3 4 1 2 3 4
Egg
Sperm

4. Gametes
All body cells except reproductive cells are called somatic cells
The somatic cells in most organisms are diploid(2n)
Gametes are haploid cells which are capable of fusion
Remember in meiosis the chromosome number is halved to ensure that fertilisation can take place
Fertilisation is the union of two gametes to form a single cell called a zygote

5. Genetic Crosses
The mechanism of genetic crosses is best understood by the use of sample questions and their answers
Explanations of new terms are provided as they appear in the following examples
Question 1
In cats, black coat (B) is dominant over white coat (b). Give the genotypes and phenotypes for the offspring of a cross involving two cats whose genotypes are (BB) and (bb)

6. Terminology Genes are represented by letters
Usually the first letter of the dominant trait is used (e.g. B)
Normally two different forms of the same gene exist, i.e. a dominant version (symbolised by a capital letter, e.g. B) and a recessive version (small letter, e.g. b).
Alleles are different forms of the same gene

7. Gene Locus
Alleles are found at the same position (or locus) on similar chromosomes
The position of a gene on a chromosome is called its locus

8. Dominant Alleles
A dominant allele prevents the working of the recessive allele
This means that when a dominant and recessive allele occur together it is the dominant allele that works
This means that cats which are BB or Bb are both black

9. Recessive Alleles
Recessive means the allele is prevented from working by the dominant allele
Recessive alleles recede in the presence of a dominant allele
This means that cats which are Bb are black, although they contain the recessive allele (b) for white coat
For a cat to be white it has to be bb

10. Genotype
Genotype means the genetic make up of an organism, i.e. the genes which are present
Normally each characteristic is controlled by a gene that has a pair of alleles
For example, the genotypes may be BB,Bb,bb

11. Phenotype
Phenotype means the physical make up, or appearance, of an organism
Referring back to question 1 the phenotype is the coat colour of the cat (i.e. the phenotype is either black or white coat)

12. Genes and the Environment
Genes are the instructions to the cell that help to produce the phenotype
Genes are also influenced by the environment
Genotype + environment = phenotype

13. Progeny Progeny refers to offspring that are produced
The F1 progeny means the first generation of offspring
F1 is short for ‘first filial generation’

14. Homozygous/Heterozygous genes
Homozygous means that two alleles are the same
Pure breeding is another term for homozygous
BB (Homozygous dominant)
bb (Homozygous recessive)
Heterozygous means that the alleles are different
Heterozygous is also called hybrid Bb

15. Working out Genetic Crosses
When working out genetic crosses you should realise that:
A pair of alleles is present in the cells of an organism for each characteristic
Only one allele for each characteristic is carried in each gamete
As a result of gametes fusing, a pair of alleles is present in the progeny

16. Answer to Question 1 B = black coat b = white coat
PARENTALPHENOTYPES black x white
PARENTALGENOTYPES
GAMETES punnett
square
F1 GENOTYPES Bb
F1 PHENOTYPES All black B B b b B b B b B b B b

17. Punnett Squares
A punnett square is a grid used to show the ratio of the genotypes of the progeny in a genetic cross

18. Incomplete Dominance
Incomplete dominance means that neither allele is dominant or recessive with respect to the other
Both alleles work in the heterozygous genotype to produce an intermediate phenotype
Incomplete dominance is also known as codominance

19. Example 1: Coat colour in shorthorn cattle RR produces a red coat, rr produces a white coat and Rr produces a roan (patches of red and patches of white)

20. Example 2: Flower colour in snapdragons
RR produces red flowers, rr produces white flowers and Rr produces pink flowers

21. Sex determination
The nucleus of each human somatic cell has 46 chromosomes (i.e. 2n = 46)
These consist of 44 non – sex chromosomes called autosomes and 2 sex chromosomes

22. The 2 sex chromosomes are called the X and Y chromosomes
They contain the genes which control gender in most species
In humans XX results in female and XY in male
It is the male who determines the sex of the child
The ratio of male to female births should be 1:1

23. Question 2: Show by means of diagrams why in humans the father determines the sex of the child
Female Male Sperm Egg Females Males X X X Y X X X Y X X X Y

24. In other species
The pattern of sex determination is some species is the opposite of that in humans
In birds, butterflies and moths males are XX and females XY

25. The Work of Gregor Mendel
Austrian Monk, born
Known as the “Father of Genetics”
Carried out numerous experiments on pea plants
His work resulted in 2 basic laws of inheritance

26. Mendel’s 1st Law of Segregation
This discovery was based on 2 main features:
He only studied features that displayed 2 characteristics. For instance the plants were either tall or small
He counted the number of plants with each type of trait. He was able to detect mathematical ratios such as 1:1 or 3:1 from the numerical data he obtained

27. “Mendel’s law of segregation states that inherited characteristics are controlled by pairs of factors. These factors will segregate from each other at gamete formation, with only one member of the pair being found in each gamete”

28. Question 3: A true breeding (now called homozygous) tall pea plant was crossed with a true breeding small pea plant. The F1 plants were allowed to self pollinate. Find the ratios of tall to small for the F1 and F2 generations

29. Do your results agree with Mendel’s findings?
Mendel repeated these experiments with other single pairs of contrasting traits and in all cases the same ratios were obtained
Mendel used this information to form his first law
Mendel’s factors are now called alleles

30. Chromosomal basis of Mendel’s first law
In diploid organisms, chromosomes occur in matching pairs called alleles
Pairs of alleles occupy the same position (locus) on a homologous pair
During meiosis, homologous chromosomes separate and go into different cells
As a result, pairs of alleles also separate

31. Homozygous dominant for tallness Homozygous recessive for tallness
Female
Male T T t t 1 1
Egg
Gametes
Sperm 1 1
F1 Genotype 1
F1 Phenotype
All Tall

32. Monohybrid and Dihybrid Crosses
“A monohybrid cross involves the study of a single characteristic such as eye colour, seed shape, or coat colour”
Each characteristic can display two variations or traits
“A dihybrid cross involves the study of 2 characteristics at the same time”

33. Mendel’s 2nd law of Independent Assortment
Mendel carried out a range of dihybrid crosses and after analysing his results formulated his Law of Independent Assortment “The Law of Independent Assortment states that when gametes are formed either of a pair of factors is equally likely to combine with either of another pair of gametes” As in the first law, the word alleles can be substituted for factors

34. Demonstration of Mendel’s Second Law
Genotype of parent Meiosis Genotypes of Gametes (all combinations are equally likely) A a B b AB Ab aB ab

35. Chromosomal basis of Mendel’s second law
As alleles are located on homologous chromosomes, Mendel’s second law can be restated as:
At gamete formation…
either of a pair of homologous chromosomes…..
is equally likely…..
to combine with either chromosome of a homologous pair

36. Question 4: In pea plants, tall plant (T) is dominant over small plant (t). In addition, green pod (G) is dominant over yellow pod (g). A tall plant with green pods (homozygous for both traits) is crossed with a small plant with yellow pods (a) Why is this a dihybrid cross? (b) Show by means of diagrams the genotypes and phenotypes of the progeny of this cross.

37. Question 5: In guinea pigs, black coat (B) is dominant to white coat (b). Also short hair (S) is dominant to long hair (s). (a) Show the genotypes and phenotypes of the F1 progeny for a cross involving a black coated, short haired guinea pig (heterozygous for both traits) and a white coated, long-haired animal. (b) State the expected ratio of the offspring.

38. Question 6: A homozygous purple-flowered, short-stemmed plant was crossed with a red-flowered, long-stemmed plant. (a) State the dominant and recessive traits (b) Explain, using diagrams, why the F1 plants all had the same phenotypes. (c) Give the expected phenotype ratios if an F1 plant is selfed.

39. Expected ratios arising from Mendel’s second law
In a dihybrid cross where one parent is heterozygous for both characteristics and the second parent is homozygous recessive, if the offspring are produced in the ratio 1:1:1:1 then independent assortment has occurred.
Equally if both parents are heterozygous for both characteristics and the offspring are in the ratio 9:3:3:1 then, again independent assortment has occurred.

40. 1Sex Linkage
The X chromosome carries a large number of genes but the Y is much shorter and carries fewer genes
The sry (sex determining region ) gene on the Y chromosome together with a small number of genes control the development of testes and therefore maleness
“Sex Linkage” means that a characteristic is controlled by a gene on an X chromosome

41. 2Sex Linkage Sex linked characteristics are also called X linked
The recessive phenotype is more likely to occur in males, i.e. males suffer more often from sex linked characteristics
Examples, are colour blindness, haemophilia (inability to form blood clots), Duchenne muscular dystrophy and and eye colour in Drosphila melanogaster
All these characteristics are controlled by genes, or alleles, located on the X chromosome
In males there is no corresponding allele on the Y chromosome

42. 1Colour Blindness
Normal individuals can detect 3 colours of light (red, green and blue)
The allele for normal vision (N) is dominant
Colour – blindness (n) usually means an inability to distinguish red from green
The gene for colour – vision is located on the X chromosome

43. 2Colour Blindness in females
Females can have 3 distinct genotypes
XNXN, XNXn, XnXn
For a female to be blind she needs 2 copies of the recessive allele
Since recessive alleles are relatively scarce, it is rare to have 2 recessives
The incidence of colour blind females in Ireland is about 0.2%

44. 3Colour Blindness in females
Males have only 1 allele for colour vision located on the X chromosome
The Y chromosome has no allele for colour vision
This means that there are only 2 genotypes for males, XNY- or XnY-
Males only need 1 recessive allele to be colour blind and are more likely to be colour blind than females
The incidence of colour blindness in Irish males is about 8%

45. Question 7
The gene for haemophilia is located on an X chromosome. Normal blood clotting (N) is dominant over haemophilia (n). Show the genotypes and phenotypes of the offspring of a cross between a mother who is a carrier and a father who is normal for the trait

46. 1Non – Nuclear Inheritance
Non nuclear circles of DNA are found in mitochondria and chloroplasts
Both of these organelles reproduce by themselves and pass on their genes to the resulting organelles
Mitochondria and chloroplasts are normally passed on to the next generation in the cytoplasm of the egg

47. 2Non – Nuclear Inheritance
Pollen do not contain these organelles
Sperm although containing mitochondria, do not pass these on
Therefore mitochondria and chloroplasts follow a maternal line of inheritance, i.e. they are inherited from the female in the cytoplasm of the egg

48. Mitochondrial Eve
Mitochondrial DNA appears to change (mutate) at a constant rate over long periods of time
This means that if mtDNA from 2 people is compared, the greater the differences, the longer ago the mtDNA diverged from a common source
Evidence from mtDNA taken from a sample of humans from different parts of the world suggests that all modern humans arose from a common ancestors who lived years ago in Africa

49. Pedigree Studies
“A pedigree study is a diagram showing the genetic history of a group of related individuals”