1. Principles of Heredity
Chapter 10

2. Gregor Mendel Generally considered the ‘Father of Modern Genetics’
Worked with pea plants, keeping careful records of his experiments
Used statistical analysis to establish several important genetic principles

3. Mendelian Genetics Things he knew before he started:
How to control pea plant reproduction
That he had true-breeding plant strains
That hybrids between these strains did NOT breed true

4. Mendelian Genetics… Things he didn’t know:
What DNA or chromosomes were
Anything about mitosis or meiosis
That traits in hybrids did not always ‘blend’ as was the idea of the time

5. Mendelian Genetics… What he figured out:
Blending of two distinct traits in the parents did not always occur in the hybrid offspring
Each kind of inherited feature in an organism is controlled by 2 factors that behave like distinct particles
That some of these factors can mask others (that is, some are dominant while others are recessive)
The Principle of Segregation and The Principle of Independent Assortment

6. The Principle of Segregation
Alleles – alternate gene forms – are located on corresponding loci on homologous chromosomes
During gamete formation in meiosis, homologous chromosomes separate (when?)
During sexual reproduction, offspring receive one of these homologous chromosomes from each parent

7. Terms to review Dominant / Recessive Gene / Allele
Monohybrid cross / Dihybrid cross
Homozygous / Heterozygous
Phenotype / Genotype
Punnett square / Probability
Test cross

8. Monohybrid cross practice
1.) A TT (tall) pea plant is crossed with a tt (short) pea plant

9. Monohybrid cross practice…
2.) A Tt pea plant is crossed with a Tt pea plant.

10. Monohybrid cross practice…
3.) A heterozygous round seeded pea plant (Rr) is crossed with a homozygous round seeded pea plant (RR).

11. Monohybrid cross practice…
4.) A homozygous round seeded pea plant is crossed with a homozygous wrinkled pea seeded plant.

12. Monohybrid cross practice…
5.) In pea plants purple flowers are dominant to white flowers. Cross two white flowered plants.

13. Monohybrid cross practice…
6.) A white flowered pea plant is crossed with a pea plant that is heterozygous for the trait.

14. Monohybrid cross practice…
7.) Two pea plants, both heterozygous for the gene that controls flower color, are crossed.

15. Monohybrid cross practice…
8.) In guinea pigs, short hair is dominant over long hair. Show the cross for a pure breeding short haired guinea pig and a long haired guinea pig.

16. Monohybrid cross practice…
9.) Show the cross for two heterozygous guinea pigs. What percentage of the offspring will have short hair? ________ What percentage of the offspring will have long hair? _______

17. Monohybrid cross practice…
10.) Two short haired guinea pigs are mated several times. Out of 100 offspring, 25 of them have long hair. What are the probable genotypes of the parents? Show the cross to prove it!

18. Dihybrid cross practice
IN PEAS:
R = round T = tall Y = yellow peas P = purple flowers
r = wrinkled t = short y = green peas p = white flowers
1.) Homozygous tall, round parent X pure short, wrinkled parent

19. Dihybrid cross practice…
IN PEAS:
R = round T = tall Y = yellow peas P = purple flowers
r = wrinkled t = short y = green peas p = white flowers
2.) Heterozygous for both height and flower color parent X short, white flowers parent

20. Dihybrid cross practice…
IN PEAS:
R = round T = tall Y = yellow peas P = purple flowers
r = wrinkled t = short y = green peas p = white flowers
3.) Green peas, short plant X Heterozygous for yellow peas, homozygous for tall parent

21. Dihybrid cross practice…
IN PEAS:
R = round T = tall Y = yellow peas P = purple flowers
r = wrinkled t = short y = green peas p = white flowers
4.) Heterozygous round, green peas X wrinkled peas, Heterozygous yellow peas

22. Dihybrid cross practice…
IN PEAS:
R = round T = tall Y = yellow peas P = purple flowers
r = wrinkled t = short y = green peas p = white flowers
5.) Both parents heterozygous for height and flower color

23. The Principle of Independent Assortment
Another of Mendel’s ideas
Explains the results of these types of dihybrid crosses
Each different trait is inherited independently from the other
Now we know that this is due to meiosis – homologous chromosomes separate independently (again, when?)

24. Probability Rules The product rule:
Predicts the combined probability of 2 independent events
If two or more events are independent of each other, the probability of both occurring is the product of their individual probabilities
Example: coin toss – heads two times in a row
½ X ½ = ¼ or one chance in 4
Also: Bb X Bb parents – producing a bb child
½ b X ½ b = ¼ or one chance in 4

25. Probability Rules… The sum rule
Predicts the combined probabilities of mutually exclusive events
If there is more than one way to get a result, we combine the probabilities by summing
Example: Bb X Bb parents chance of Bb child
2 possibilities: B egg + b sperm; b egg + B sperm
B egg (½) X b sperm (½) = ¼
And b egg (½) X B sperm (½) = ¼
Then … ¼ + ¼ = ½

26. Probability Rules… Most important – Chance has no memory!
If events are truly independent, past events have no influence on the probability of future events…
Even though we don’t like this idea – ‘my luck is bound to change…’

27. Things Mendel didn’t know
Linked genes - inherited together because they are located on the same chromosome
Linked genes do not undergo segregation or independent assortment
The rates of crossing over can be used to determine the relative positions of genes on a chromosome
Higher crossing over rates indicate greater separation of genes on a chromosome
Each %age of crossing over rate = one map unit

28. Things Mendel didn’t know…
Sex chromosomes:
Female = XX - Male = XY
Male produces the sex determining gamete
In humans, Y chromosome has the SRY gene (sex reversal gene on the Y) – this acts as a genetic switch to cause testes to develop
Developing testes produce testosterone which determines other sexual characteristics
Everyone has at least one X – female is the ‘default’ sex… need a Y to develop as a male

29. Things Mendel didn’t know…
Sex-linked genes:
Located on the X chromosome only
Include genes for color perception and blood clotting – things all humans need
Females get two copies – can be either homozygous or heterozygous
Males only get one copy – they are hemizygous
Defects in these traits arise more in males than in females

31. Things Mendel didn’t know…
Dosage compensation:
Makes equivalent the female’s 2 ‘doses’ of the genes on the X chromosome to the male’s 1
One X chromosome in each female cell is inactivated – called a Barr body
Individuals with heterozygous X-linked genes will often have a variegated phenotype as random X chromosomes are inactivated in the body

32. Calico cats – an example of X chromosome inactivation in action

33. Things Mendel didn’t know…
Incomplete dominance
The phenotype of the heterozygous individual is a blending of the two genes

34. Things Mendel didn’t know…
Codominance:
The phenotype of the heterozygous individual expresses both genes, but without blending
ABO Blood Types:
A and B alleles are codominant to each other
Both are dominant to type O allele

35. Things Mendel didn’t know…
Multiple alleles:
Three or more alleles exist in the population, even though each individual only has two
Examples:
ABO blood types in humans
Coat color in rabbits

36. Things Mendel didn’t know…
Pleiotrophy
One gene with many effects
Often found in genetic diseases
Example:
Cystic fibrosis in humans
Homozygous individuals produce abnormally thick mucus in many body systems

37. Things Mendel didn’t know…
Epistasis
The presence of one allele can prevent or mask the expression of a gene at another loci
Example:
Coat color in Labrador retrievers
Pigment gene is either B (black) or b (brown)
Recessive ee blocks the expression of either
Black Brown Yellow
BBEE bbEE BBee
BbEE bbEe Bbee
BBEe bbee
BbEe

38. Things Mendel didn’t know…
Polygenic inheritance
Mulitple independent pairs of genes have similar and additive effects on the phenotype
The phenotypes in a population will generally show a normal distribution curve
Examples:
Human skin and eye color

39. Things Mendel didn’t know…
Environmental interaction
Genetically identical individuals show different phenotypes based on environmental factors
Example:
Human height and intelligence
Nature versus nurture questions
Problems with experimental methods to answer these questions in humans….