1. Genetics Heredity – the passing of traits from parent to offspring
Genetics- the study of heredity
Part I: Mendel and the Gene Idea

2. I. Gregor Mendel He was a monk, gardener, mathematician
1st to apply statistics to selective breeding
Published work on pea plant inheritance patterns in the 1860’s.
(nothing known about the
cell for inheritance…)

3. Why was the pea a good choice?
1) Cheap and available
2) Produce offspring quickly/short generation time
3) Characteristics are determined on 1 gene with only 2 alleles.
Ex. Flower color: purple or white
4) No trait blending
5) Can control breeding. (Easy access to male and female sex parts)

5. Mendel’s Questions
Mendel crossed (bred) two different plants to discover what traits the offspring would show.
Ex: Will a purple flower plant crossed with a purple flower plant produce all purple offspring?

6. II. Basic genetic concepts
A. Mendel studied 7 different characters in peas:
Ex)
Height: tall vs. dwarf
Seed shape: smooth vs. wrinkled

7. Characters – inherited features of an organism.
Traits – variations of a character.
Ex) character:
fur color
possible traits:
brown, black, red

8. Review of chromosomes A. Each = one DNA molecule
B. Gene – DNA sequence found on a particular chromosome, that codes for a particular trait.
C. Each may have thousands of genes for a particular set of genes
Ex. Human chromosome 11 has the genes for making the enzymes hemoglobin, catalase, and insulin (and thousands of other genes)

9. Alleles are possible forms of the same gene.
Genes express alleles
Alleles are possible forms of the same gene.
Eye color
brown
blue

10. More about alleles:
Dominant allele – the “stronger” allele. In a heterozygous combination the dominant allele will be expressed.
Symbol is a capital letter
Ex) trait: pea plant height
tall is dominant, tall = T
Recessive allele – “weaker” allele. In the heterozygous individual these are hidden, and the dominant form of the trait will be expressed.
Symbol is a lowercase letter
dwarf is recessive, dwarf = t

11. Allele combinations for genes
HOMOZYGOUS – both gene are the same.
Ex) Seed coat trait:
wrinkled seed allele
and wrinkled seed allele
HETEROZYGOUS – genes are different
Ex) seed coat trait
wrinkled seed allele and round seed allele f

12. Genotype: the alleles an individual has
Phenotype: the way those alleles are expressed
Ex) pea plant height
Possible genotypes: T T, T t, or t t
Possible phenotypes: tall or dwarf
Possible combinations:
Homozygous dominant = TT, tall
Homozygous recessive = tt, dwarf
Heterozygous = Tt, tall

13. Example of an individual’s allele combinations on three gene loci

14. Mendel’s Experiments
True-breeder – an individual that always produces offspring with the same expression for a given trait.
Ex) purple flower plant always produces purple flower offspring
P1 – the parent generation. Two true breeders with different traits for the same character are crossed
Ex) true breeding purple flower pea X true breeding white flower pea
F1 – first filial generation. The offspring (progeny) of the P1
F2 – second filial generation. The offspring of two individuals from the F1 generation.

15. Experiment Results

16. C. Mendel’s Conclusions
Mendel’s Rules of Inheritance – generalizations made by Mendel
Law of Segregation
Law of Independent Assortment

17. Mendel’s Rules of Inheritance
Different versions of genes account for variety in organisms.
2) For each character, an organism inherits two alleles, one from each parent.
3) If two alleles differ, then one (dominant) is fully expressed, and the other (recessive) has no effect.

18. Mendel’s Laws of Inheritance
Law of Segregation - The two alleles a parent has for each character segregate during gamete production.
Ex. Heterozygote pea for flower color
Pp : half of gametes get P, other half get p
Law of Independent Assortment – for each gene the alleles separate independently of alleles for other genes
Ex) heterozygote for flower color (Pp) and seed color (Gg)
some gametes will get PG, some will get Pg, pG, or pg, in a ¼ ratio
Refers to two or more genes/characters

19. A Test Cross
WHY? To determine whether an organism that has the dominant trait is homozygous dominant or heterozygous.
HOW? Cross with a homozygous recessive organism. Does the recessive phenotype show up?
How to read the results?
100% dominant offspring = homozygous dominant parent
ANY recessive offspring = heterozygous parent.

20. Patterns of Inheritance NOT revealed by Mendel’s studies
Incomplete dominance
Codominance
Multiple alleles
Polygenic traits

21. Incomplete dominance
In heterozygotes both alleles are expressed so the trait blends.
Example: flower color in snapdragons
allele F R = red
allele F W = white
In heterozygotes (F R F W) = pink

22. Codominance
In heterozygotes both alleles are expressed in separate distinguishable ways
Example: Roan horse
RR – there are red hairs
rr – there are white hairs
Rr – some hairs are
white and some are red

23. Multiple alleles
There are more than two possible alleles for a character.
Example: human ABO blood group
3 alleles A, B, and O (allele = O)
O is recessive, A and B are codominant
4 possible phenotypes
Type A blood (genotypes: AA or AO)
Type B blood (BB or BO)
Type O blood (OO)
Type AB blood (AB)

24. Human Blood Types

25. Polygenic inheritance
-many genes affect one phenotype.
Ex) human skin pigmentation

26. Sex-linked genes
– genes that are on the X chromosome show unique inheritance patterns
The Y chromosome (ONLY male gender). The X chromosome has other genes on it.
Sex-linked traits
can pass from mom to sons OR daughters
can pass from the dad to ONLY daughters

27. Mendelian Inheritance in humans: - from dominanat/recessive alleles on one gene.
Examples:
Widow’s peak
Attached or free earlobes
Recessive disorders*: Ex.s) cystic fibrosis, Tay-Sachs disease, sickle-cell disease
Dominant disorders*: Ex.) Huntington’s
*Determined with pedigree analysis, genetic testing and counseling