1. Topic 4.3
Theoretical
genetics

2. 4.3 Define…

3. Mendel’s experiments

4. Mendel’s experiments
Monohybrid cross: A cross that tracks the inheritance of a single character.
Pea colour Mendel’s experiment
Flower colour Mendel’s experiment:
25%
white-flower peas F2
75%
purple-flower peas
3:1
true-breeding P
100% F1
self-pollinate
When Mendel crossed true-breeding plants with different character traits, he found that the traits did not blend.
Using the scientific process, Mendel designed experiments in which he used large sample sizes and kept accurate quantitative records of the results.
For example, a cross between true-breeding varieties, one with purple flowers and one with white flowers produced F1 progeny (offspring)with only purple flowers.
Mendel hypothesized that if the inheritable factor for white flowers had been lost, then a cross between F1 plants should produce only purple flowered plants. When Mendel allowed the F1 plants to self-fertilize (self-pollinate). Based on a large sample size, Mendel recorded 705 purple-flowered and 224 white-flowered in the F2 generation a ratio of 3:1 The inheritable factor for white flowers was not lost (reappeared in the F2). , so the hypothesis was rejected. From these types of experiments and observations, Mendel concluded that since the inheritable factor for white flowers was not lost in the F1 generation, it must have been masked by the presence of the purple flowers factor.

5. Mendel’s experiments
Mendel reasoned that the heritable factor for white flowers was present in the F1 plants, but it did not affect flower color. Purple flower is a dominant trait and white flower is a recessive trait. The hybrid plants inherit one factor from each parent and so have two different factors. When they form male and female gametes they pass on either the dominant or the recessive factor, not both. The factors therefore separate when male and female gametes are made. This is called segregation.
Therefore, the Principle of Segregation indeed is a general principle of genetics.

6. Mendel’s conclusions:
Inheritance is not blended.
The inheritable factor for white color was not lost, it was masked by the presence of purple color factor.
The hybrid plants inherit one factor from each parent and so have two different factors.
When they form male and female gametes they pass on either the dominant or the recessive factor, not both.
Mendel’s Law of Segregation:
Each genetic trait is produced by a pair of alleles which separate during reproduction.
During meiosis, each gamete only gets one copy of the genetic trait.

7. Mendel’s conclusions:
Mendel’s Law of Segregation:
Mendel’s factors are now called alleles and the character of an organism is carefully distinguished from its alleles using the terms genotype and phenotype.

8. 4.3.2 Punnet Squares (1) Define symbols: P = Purple allele
p = White allele
(2) State the cross
(3) Diagram the gametes
(4) Complete the squares
(5) Summarize the results:
Genotype
Phenotype
In a Punnett square for a monohybrid cross, the Principle of Segregation is applied.

9. Monohybrid test cross
How can you determine genotype from individual expressing dominant phenotype? - DD or Dd?
Cross the individual with dominant phenotype with a homozygous recessive individual.

10. 4.3.2 Punnet Squares

11. Others inheritance pattern
Co-dominance
Multiple alleles: ABO group
Important note: Co-dominance is not the same than blending inheritance
Some genes have more than two alleles. Blood groups is an example of codominance as well as multiple alleles. Both alleles are expressed in blood types.
Three blood type alleles: i, IA, IB
The alleles for blood types A and B are codominant

12. 4.3.5 Determination of gender in humans
Y chromosome carries genes for male development.
XX = female
XY = male
Male gamete determines gender of offspring
Egg always carries a single X
Sperm carries either X or Y
How many chromosomes do humans have?
How many pairs of chromosomes?
How many of them are autosomes?
How many of them are sex chromosomes ?

13. 4.3.5 Determination of gender in humans
Some genes are present on the X chromosome and absent from the shorter Y chromosome in humans

14. 4.3.7 Define sex linkage ♀:2 X’s Chromosomes ♂:1 X Chromosome
Genes located on a sex chromosome are called sex linked genes.
In humans the term usually refers to X-linked characters: genes located only on X chromosomes.
Fathers can pass X-linked alleles to their daughters, but not sons.
Mothers can pass sex-linked alleles to both sons and daughters.
Homozygous or Heterozygous
for a sex-linked trait
♀:2 X’s Chromosomes
Hemizygous
for a sex-linked trait
Shows a recessive trait
even with only one recessive allele
♂:1 X Chromosome

15. 4.3.8 Describe the inheritance of colour blindness
The allele of color vision is carried on the part of the X chromosome that is missing from the Y chromosome. This means that a male will only have one allele for color vision.
There is a defective, recessive allele of the color vision gene which can lead to color blindness, particularly of red and green light.
C= allele for normal vision
c= allele for color blindness
What happens if we cross a normal sighted female with a color blind male?
Females are carried of one allele do not show it in their phenotype
What happens if we cross a normal sighted male with a carrier female?
There is a 1 in 4 chance that the carrier female will pass on the color-blind to one of her children and it will always be a boy

16. 4.3.8 Describe the inheritance of colour blindness
What happens if we cross
a normal sighted female with
a color blind male?
What happens if we cross
a normal sighted male with
a carrier female?

17. 4.3.8 Describe the inheritance of Haemophilia
Haemophilia is a sex-linked disease, caused by a recessive allele on the X chromosome.
The gene that codes for Factor VIII, an important protein involved in blood clotting, is a sex-linked gene found on the X chromosome.
A defective recessive allele of the gene can lead to haemophilia.
H= normal allele
h= haemophiliac allele
The gene that codes for Factor VIII, an important protein involved in blood clotting, is a sex-linked gene found on the X chromosome.
Haemophilics are unable to make Factor VIII and can lose a lot of blood from even small injuries or bruises. In the past this meant that most sufferers died in chilhood, but now can be treated by regular injections of Factor VIII.
Can you see that the male only has to have one recessive allele to get haemophilia? This is because its effects cannot be masked by a dominant, normal allele on the Y chromosome.
Haemophiliac females occurs very rarely. Why do you think that the onset of menstruation at puberty can often prove fatal for female haemophiliacs?

18. 4.3.10 Females carriers for X-linked recessive alleles.
4.3.9 Female and sex linkage
 Females can be homozygous or heterozygous for the sex-linked alleles
  Females carriers for X-linked recessive alleles.
Carrier are individuals that are heterozygous for the allele.
The have both the dominant and the recessive (disease) allele.
Carriers do not have the disease.

19. 4.3.11 Predict genotypic and phenotypic ratios…

20. Pedigree Charts:
Pedigree charts show a record of the family of an individual
They can be used to study the transmission of a hereditary condition
They are particularly useful when there are large families and a good family record over several generations.

21. Pedigree Charts: I II III IV
Generations are identified by Roman numerals
Individuals in each generation are identified by Arabic numerals numbered from the left
Therefore the affected individuals are II3, IV2 and IV3 I II
III IV

22. 4.3.11 Deduce the genotypes and phenotypes …
Females who have more than four sons, with none exhibiting hemophilia, are likely to have the genotype NN. If she has had four or fewer sons her genotype is less certain. In such cases her genotype is labeled N_. LABEL THE REST OF THE FEMALES AS EITHER N? OR NN.
Q10 What is the probability of parents 1 and 2 having a hemophiliac child ?
Q11 Is there any chance of parents 3 and 4 having a hemophiliac son ? Explain.
Figure I Hemophilia Pedigree

23. 4.3.11 Predict genotypic and phenotypic ratios…
Hemophilia has played an important role in Europe’s history, for it suddenly cropped up in the children of Great Britain’s Queen Victoria. It became known as the “Royal disease” because it spread to the royal families of Europe through Victoria’s descendants