1. Chpt. 17
Genetic Crosses

2. Gregor Mendal is known as the father of genetics!!!

3. General Definitions Genetics: is the study of inheritance.
Somatic Cells: are all cells except sex cells
e.g. Cheek, liver, muscle, blood, leaf, stem
etc.
Gametes (sex cells): are haploid cells that
are capable of fusion.
Fertilisation: is the union of two gametes
to form a single called a zygote.

4. Genetic Crosses – Definitions
Alleles: are different forms of the same
gene.
Eg. Rabbits may have long ears (L) or short ears (l). So, the gene for ears has two alleles L or l.
Dominant: means that the allele prevents
the recessive allele working.

5. Genetic Crosses – Definitions
Examples of dominant traits:
Genes for
Brown eyes
Ear lobes
Dimples in chin
Freckles
Curly hair
Tongue rolling

6. Genetic Crosses - Definitions
Recessive: means that the allele is
prevented from working by a dominant
allele.
Examples include:
- Cystic fibrosis
- Albinism
- Haemophilia
Eg. A rabbit with the alleles Ll has long ears because the recessive allele for short ears (l) is prevented from working by the dominant allele for long ears (L).

7. Genetic Crosses – Definitions
Genotype: means the genetic make up of an
organism, i.e. the genes that are present.
E.g. The possible genotypes for ear lengths
in rabbits are LL, Ll, ll

8. Genetic Crosses – Definitions
Phenotype: means the physical make-up or
appearance, of an organism.
- Eg. The phenotypes of the rabbits are either long ears or short ears.
- It is the interaction of the genes
(genotype) with the environment that produces the phenotype:
Phenotype = Genotype + Environment

9. Genetic Crosses – Definitions
Homzygous (Pure Breeding): means that both alleles
are the same. i.e. The genotypes LL and ll are both
homozygous.

10. Genetic Crosses - Definitions
Heterozygous (Hybrid): means that the alleles are
different. i.e. the genotype Ll is heterozygous.

11. Genetic Crosses - Definitions
Progeny: refers to offspring that are
produced:
F1 progeny means the first generation of
offspring (first filial generation).

12. Genetic Crosses Points to note:
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.

13. Example 1:

14. Example 2:

15. Genetic Crosses – Questions
In rabbits, long ears(L) are dominant over short ears (l). Show the genotypes and phenotypes for the offspring of a cross between two rabbits whose genotypes are (LL) and (ll).
Solution 1:
Parents Genotypes LL x ll
Genotypes of Gametes L x l
F1 Genotype of Offspring Ll
F1 Phenotype of offspring All long ears

16. Genetic Crosses – Questions
When answering genetic questions a punnett square is used for the more difficult questions.
Punnett Square: is a grid used to show the ratio of the genotypes of the first generation of offspring (progeny) in a genetic cross.
Six sample questions will be completed involving the use of a Punnett Square (see attached leaflet)

17. Incomplete Dominance
Incomplete dominance (codominance): means that neither allele is dominant or recessive. Both alleles work in the heterozygous condition to produce an intermediate phenotype.
Incomplete dominance is relatively rare.
Example 1:
In shorthorn cattle:
- the genotype RR produces a red coat
- the genotype rr produces a white coat
- The heterozygous genotype Rr gives a roan coat ( patches of red and patches of white)

19. Incomplete Dominance Example 2: In snapdragons (flower):
- the genotype RR produces red flowers
- the genotype rr produces white flowers
- the heterozygous genotype Rr produces pink flowers

20. Example 2:

21. Example 2 Contd.:

22. Pedigree Studies
Pedigree: is a diagram showing the genetic history of a group of related individuals.
See pg. 168 Question 7

23. Sex Determination Normal body cells in humans contain 46 chromosomes:
- 44 chromosomes – have genes that do not control sexuality – called autosomes.
- 2 chromosomes – control sexuality – heterosomes.
Autosomes – control features independent of whether a person is male or female:
- skin colour
- number of arms
- formation of saliva production of digestive enzymes

24. Sex Determination
Heterosomes - these two chromosomes are the X and Y chromosomes.
- male body cells are XY
- female body cells are XX
The arrangement of XX for females and XY for males has the following two consequences:
- it is the male who determines the sex of the child.
- the ratio of male to female births should be 1:1.

26. Sex Determination in Other Species
The pattern of sex determination in some species is the reverse of that in humans.
For example in birds, moths and butterflies:
- male body cells – XX
- female body cells – XY
As a result of the this it is the female who determines the sex of the offspring.

27. Mendel’s Experiments (Higher Level)
Gregor Mendel – ‘The Father of Genetics’
Research carried out around 1860 but ignored until
1900’s
Mendel’s work resulted in two basic laws of
inheirtance – ‘Mendel’s 1st and 2nd laws’

28. Pair of gametes each having only one allele
Mendel’s 1st Law – Law of Segregation
Inherited traits are controlled by two alleles
The alleles separate at gamete formation, with each
gamete only having one allele
Example:
Consider a species of plant whose height is controlled by a pair of alleles Tt. When gametes are formed only one allele can enter each gamete i.e. the alleles segregate T Tt
Pair of gametes each having only one allele t

29. Each cell has only one chromosome and one allele
Mendel’s 1st Law – Law of Segregation
Behaviour of chromosomes according to Mendel’s 1st law:
Diploid organisms – chromosomes occur in
homologous pairs.
Pairs of alleles occupy same position on a homologous
pair.
During meiosis, homologous chromosomes separate
and go into different cells.
As a result pairs of alleles also separate T
Each cell has only one chromosome and one allele T t
Meiosis t

30. Monohybrid Cross: involves the study of a single characteristic e. g
Monohybrid Cross: involves the study of a single characteristic e.g. eye colour, seed shape etc.
Dihybrid Cross: involves the study of two characteristics at the same time e.g. studying plant size (tall or small) and pod colour (green or yellow) at the same time.

31. Mendel’s 2nd Law – ‘Law of Independent Assortment’
When gametes are formed .....
either of a pair of alleles
is equally likely
to combine with either of another pair of alleles
Example:
AaBb
A can combine with either B or b
a can combine with either B or b
Four types of gamete can form
AB Ab aB ab

32. Mendel’s 2nd Law – ‘Law of Independent Assortment’
Behaviour of chromosomes according to Mendel’s 2nd Law:
At gamete formation...
either of a pair of homologous chromosomes....
is equally likely
to combine with either chromosome of a second
homologous pair

35. Dihybrid Crosses Example 1:
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.
Why is this a dihybrid cross?
Show by diagrams the genotypes and phenotypes of the progeny of this cross.

36. Example 2:
In guinea pigs, black coat (B) is dominant to white coat (b). Also short hair (S) is dominant to long hair (s).
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.
State the expected ratio of the offspring

37. Example 3:
A homozygous purple-flowered, short stemmed plant was crossed with a red flowered, long stemmed plant.
All the F1 offspring were purple-flowered with short stems.
State the dominant and recessive traits.
Explain, using diagrams, why the F1 plants all had the same phenotypes
Give the expected ratios if an F1 plant is selfed.

38. Independent Assortment has occurred!!!
Mendel’s 2nd Law – Expected Ratios
Case 1 (Dihybrid Cross):
Heterozygous X Homozygous Recessive
(both characteristics)
Ratio of offspring = 1 : 1 : 1 : 1
Independent Assortment has occurred!!!
Case 2 (Dihybrid Cross):
Heterozygous X Heterozygous
(both characteristics) (both characteristics)
Ratio of offspring = 9 : 3 : 3 : 1

39. Linkage Means that both genes are present on the same chromosome
Linked genes are passed on together
Linked Genes
R and S
r and s
No Linkage
T and t r R T t s S
Pair of alleles
Homologous Pair

40. *Note: The position a genes occupies on a chromosome
Linkage
Linkage contradicts Mendel’s 2nd law of independent
assortment (see following examples)
Much less variation in offspring
*Note: The position a genes occupies on a chromosome
is called the locus

43. Linkage Example 1:
Draw simple chromosome diagrams to illustrate the following cells. In each case show the gametes that might be produced.
a) The genes are not linked and the genotype is AaBb.
b) The genes are linked (A to B and a to b) and the genotype is AaBb.

44. Linkage Example 2:
Draw simple chromosome diagrams to show each of the following cells. In each case indicate the gametes that each cell might produce.
a) Genotype RrSs, genes are not linked
b) Genotype RrSs, genes are linked (R to S and r to s)
c) The genes are not linked, the cell is homozygous for R and heterozygous for S
d) The genes are linked, and the cell is homozygous dominant for both genes.

45. Linkage Example 3:
Show the expected genotypes of the progeny for the following crosses, AaBb x aabb:
a) where there is no linkage
b) where the genes are linked, A to B and a to b

46. Note: in males there is no corresponding allele on the Y chromosome
Sex Linked (X Linked)
Sex chromosomes in humans:
X – carries a large number of genes
Y – much shorter than X and carries
very few genes
Sex linkage means that a characteristic is controlled by
a gene on an X chromosome.
Examples of traits controlled by a gene on the X
chromosome:
Colour blindness
Haemophilia
Duchenne muscular dystrophy
In sex linked characteristics, the recessive phenotype is
more likely to occur in males i.e. males suffer more
often from sex linked characteristics.
Note: in males there is no corresponding allele on the Y chromosome

47. Sex Linked (X Linked)
Females tend to be carriers - they carry the gene for
the trait but do not suffer from the disease

48. Carrier Mother & a Colour Blind Father
Parents Genotype NXXn nXY
Gametes NX nX nX Y
Offspring Genotype NXnX NXY nXnX nXY
Offspring Phenotype:
female male female male
carrier normal CB CB

49. Colour-blindness A colour-blind female inherits the colour-blind gene
from her mother( a carrier) as well as from her father -
both parents must have the gene.
For a boy to be colour-blind, it is necessary only that his
mother is a carrier. This is far more common and the
reason why far more boys are colour-blind than girls

50. Haemophilia Haemophilia – is a bleeding disorder caused by the
lack of a particular blood protein
Caused by a gene located on the X chromosome:
- allele (N) for production of clotting protein is dominant.
- recessive allele (n) does not carry the correct genetic code for the production of the protein.
More common in males, extremely rare in females

51. Non-nuclear Inheritance
Extra-nuclear genes are present as small circles of
DNA in mitochondria and chloroplasts (both of which
reproduce by themselves passing on their genes)
Since, pollen does not contain these organelles and
mitochondria are in the tail of the sperm, only the
head joins with the egg, this means that
mitochondria and chloroplasts are inherited from
the female in the cytoplasm of the egg.
Mitochondrial DNA (mtDNA) in humans is inherited
only from the mother, subsequently, a number of rare
human disorders are inherited only from the mother
because they are controlled by extra-nuclear genes
located on mtDNA.

52. Mitochondrial Eve
Please read over paragraph relating to this topic pg. 179 in your book.