1. GENETIC INHERITANCE

2. Lesson Objectives At the end of this lesson you should be able to
Give a definition for a gamete
Understand gamete formation
Give the function of gamete in sexual reproduction
Define fertilisation
Define allele
Differentiate between the terms homozygous and heterozygous

3. Lesson Objectives (cont.)
At the end of this lesson you should be able to
Differentiate between genotype and phenotype
Differentiate between dominant and recessive
Show the inheritance to the F1 generation in a cross involving:
Homozygous parents
Heterozygous parents
Sex determination
Show the genotypes of parents, gametes and offspring

4. Sexual Reproduction Involves two parents
Each parent makes reproductive cells
- called gametes

6. Outline of Fertilisation
Gametes join together by fertilisation
Form a diploid zygote
This develops into an embryo
Eventually into a new individual
New individual resembles both parents – but is not identical to either

7. What are Gametes? Reproductive Cells Formed by meiosis
Contain single sets of chromosomes
- haploid
Capable of fusion to form zygote
- diploid
Zygote contains genetic information of both gametes

8. Sex Determination

9. Human Chromosomes We have 46 chromosomes, or 23 pairs.
44 of them are called autosomes and are numbered 1 through 22. Chromosome 1 is the longest, 22 is the shortest.
The other 2 chromosomes are the sex chromosomes: the X chromosome and the Y chromosome.
Males have and X and a Y; females have 2 X’s: XY vs. XX.

10. Male Karyotype

11. Female Karyotype

12. Sex Determination
The basic rule: If the Y chromosome is present, the person is male. If absent, the person is female.

13. Meiosis
the X and Y chromosomes separate and go into different sperm cells:
½ the sperm carry the X and the other half carry the Y.
All eggs have one of the mother’s X chromosomes
The Y chromosome has the main sex-determining gene on it, called SRY

14. Sex Determination
About 4 weeks after fertilization, an embryo that contains the SRY gene develops testes, the primary male sex organ.
The testes secrete the hormone testosterone.
Testosterone signals the other cells of the embryo to develop in the male pattern.

15. Genetics The study of heredity.
Gregor Mendel (1860’s) discovered the fundamental principles of genetics by breeding garden peas.

16. Genetic Terms - Alleles
Alternative forms of genes.
Units that determine heritable traits.
Dominant alleles (TT - tall pea plants)
a. homozygous dominant
Recessive alleles (tt - dwarf pea plants)
a. homozygous recessive
Heterozygous (Tt - tall pea plants)

18. Phenotype Outward appearance Physical characteristics Examples:
1. tall pea plant
2. dwarf pea plant

19. Arrangement of genes that produces the phenotype
Genotype
Arrangement of genes that produces the phenotype
Example:
1. tall pea plant
TT = tall (homozygous dominant)
2. dwarf pea plant
tt = dwarf (homozygous recessive)
3. tall pea plant
Tt = tall (heterozygous)

20. A Punnett square is used to show the possible combinations of gametes.

21. Breed the P generation
tall (TT) vs. dwarf (tt) pea plants T t

22. tall (TT) vs. dwarf (tt) pea plants
All Tt = tall
(heterozygous tall)
produces the
F1 generation Tt

23. Breed the F1 generation
tall (Tt) vs. tall (Tt) pea plants T t T t

24. tall (Tt) vs. tall (Tt) pea plants
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype TT Tt tt

25. Monohybrid Cross
A breeding experiment that tracks the inheritance of a single trait.
Mendel’s “principle of segregation”
a. pairs of genes separate during gamete formation (meiosis).
b. the fusion of gametes at fertilization pairs genes once again.

26. Homologous Chromosomes
eye color locus
B = brown eyes
eye color locus
b = blue eyes
This person would have brown eyes (Bb)
Paternal
Maternal

27. Meiosis - eye color B B Bb b b sperm haploid (n) diploid (2n)
meiosis II B b
meiosis I Bb
diploid (2n)

28. Monohybrid Cross
Example: Cross between two heterozygotes for brown eyes (Bb)
BB = brown eyes
Bb = brown eyes
bb = blue eyes B b
Bb x Bb
male
gametes
female gametes

29. Monohybrid Cross B b Bb x Bb 1/4 = BB - brown eyed
1/4 = bb - blue eyed
1:2:1 genotype
3:1 phenotype BB Bb bb

30. Dihybrid A genetic cross where two contrasting traits are investigated
Eg: TtYy or TTYY

31. Law of Independent Assortment (mendels 2nd law)
When gametes are formed, each member of a pair of alleles may combine randomly with either of another pair (if genes are not linked)

32. The allele for tongue rolling (R) is dominant to the allele for non tongue rolling (r). Also the allele for brown hair (B) is dominant to red hair (b). Neither of these characteristics is sex linked.
Using the punnet square determine the possible F1 generation genotypes of a cross between two heterozygous parents (heterozygous for both characteristics).

33. In the fruit fly, Drosophila, the allele for grey body (G) is dominant to the allele for ebony body (g) and the allele for long wings (L) is dominant to the allele for vestigial wings (l). These two pairs of alleles are located on different chromosome pairs.
(i) Determine all the possible genotypes and phenotypes of the progeny of the following cross: grey body, long wings (heterozygous for both) X ebony body, vestigial wings.

34. Incomplete Dominance
Niether genes are dominant over the other and form an intermediate phenotype in the heterozygous offspring.
Example: snapdragons (flower)
red (RR) x white (rr)
RR = red flower
rr = white flower R r

35. Incomplete Dominance R produces the Rr F1 generation r All Rr = pink
(heterozygous pink)
produces the
F1 generation Rr

36. Pink Flowers?

38. Co-dominance
Both alleles are dominant and both express their phenotype (multiple alleles) in heterozygous individuals.
Example: blood
1. type A = IAIA or IAi
2. type B = IBIB or IBi
3. type AB = IAIB
4. type O = ii

39. Co-dominance Example: homozygous male B (IBIB)
x heterozygous female A (IAi) IA IB i
IAIB
IBi
1/2 = IAIB
1/2 = IBi

44. Practice with Crosses http://www.zerobio.com/drag_gr11/mono.htm

45. Chromosomes and Genetics
Chromosomes are long pieces of DNA, with supporting proteins
Genes are short regions of this DNA that hold the information needed to build and maintain the body
Genes have fixed locations: each gene is in a particular place on a particular chromosome
Diploids have 2 copies of each chromosome, one from each parent. This means 2 copies of each gene.

46. Linkage
genes are genes located on the same chromosome, which tend to be inherited together.

47. Example: Drosophila fruit fly:
Genes for body colour and wing length are on the one chromosome i.e. are linked.
Grey body (G) and long wings (L) are dominant to black body (g) and vestigial wings (l). G with L and g with l  
Parents: GGLL X ggll

48. G G g g
L L l l
Gametes: GL X gl
G g
L l
F1: GgLl

49. Self-cross (if genes linked):
Parents: GgLl X GgLl
Gametes: GL gl GL gl
F2: GGLL GgLl GgLl ggll

50. Sex determination
Autosome: a chromosome other than the sex chromosomes.
Sex chromosomes: chromosomes that determine the sex of an individual - XX or XY.
   

51. Parents: Mother X Father
X X X Y
Gametes: X (Egg) X orY (Sperm)
F1 genotype: XX XY
Phenotype: Female Male

52. The male thus determines the sex of an offspring.
Mother gives an X to everyone but father gives an X or Y chromosome. There is a 50:50 chance that any child will be male/female)

55. In man sex-linked genes (i. e
In man sex-linked genes (i.e. those on the X chromosome with no corresponding part on the Y chromosome) include those governing red-green colour blindness, muscular dystrophy and haemophilia (inability to clot blood).
Females with both recessive genes for haemophilia do not survive beyond the first four months of gestation period.

56. Parents: Female carrier X Male normal
XHXh XHY
Gametes: XH Xh XH Y

57. F1 XHXH XHY XHXh XhY
Female Male Female Male
Normal Normal Carrier Haemophili
25% chance of producing a haemophiliac child
50% chance of producing a haemophiliac son.
It is the mother that determines if the son is haemophiliac or not since the father always passes the Y chromosome to his son.

58. Colour blindness is caused by a recessive gene on the X chromosome.
Parents:
Female carrier X Male colour blind

59. End