1. Mendel & The Gene Idea Chapter 14

2. Mendelian Genetics Character Trait (allele) True-bred (purebred)
heritable feature; ex - fur color
Trait (allele)
variant for a character; ex- brown
True-bred (purebred)
homozygous for a trait
Hybridization
crossing of 2 different purebreds; ex – purple with white
P generation
parents
F1 generation
first filial generation; filial = “son”; product of P generation
F2 generation
product of crossing two from the F1 generation

3. Genetics vocabulary…….
Punnett square: predicts the results of a genetic cross between individuals of a known genotype
Homozygous: pair of identical alleles for a character
Heterozygous: two different alleles for a character
Phenotype: an organism’s physical appearance
Genotype: an organism’s genetic makeup (allele combination)
Testcross: breeding of a recessive homozygote with a dominate phenotype (but unknown genotype)

4. The 3:1 Ratio: What Must Be True
Alternative versions of genes (alleles) account for variations in inherited characters
For each character, an organism inherits 2 alleles, one from each parent
If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance
The alleles for each character segregate (separate) during gamete production (meiosis) ending up in different gametes = Mendel’s Law of Segregation

5. Mendel’s Law of Independent Assortment
Mendel derived the Law of Segregation from following a single allele at a time (monohybrid crosses)
Mendel’s second law came about when he was following two alleles at a time (dihybrid crosses)
The segregation of one character does not influence the segregation of another character during gamete formation = Mendel’s Law of Independent Assortment

6. Practice
A pea plant heterozygous for inflated pods (Ii) is crossed with a plant homozygous for constricted pods (ii). Draw a punnett square for this cross and determine the genotypic and phenotypic ratios.
Pea plants heterozygous for flower position (AaTt) are allowed to self-pollinate, and 400 of the resulting seeds were planted. Draw a punnett square for this cross. How many offspring would be predicted to have terminal flowers and be dwarf? See table 14.1, pg 254 (7th ed)or 265 (8th ed)

7. Probability: Multiplication Rule
What is the probability that the offspring will be homozygous recessive (rr)?
Multiply the probability of the egg having an ‘r’ by the probability of the sperm having an ‘r’
½ x ½ = ¼

8. Probability: Addition Rule
What is the probability the offspring will be heterozygous?
There are 2 ways to be heterozygous: Rr or rR.
Calculate the probabilities of getting Rr and rR and then add them together
¼ + ¼ = ½

9. Example Using Probabilities
What fraction of offspring from the cross below would be predicted to exhibit the dominant phenotypes for both of the characters?
YyRr x Yyrr

10. Practice
For any gene with a dominant allele C and a recessive allele c, what proportions of the offspring from a CC x Cc cross are expected to be homozygous dominant, homozygous recessive, and heterozygous?

11. An organism with the genotype BbDD is mated to one with the genotype BBDd. Assuming independent assortment of these two genes, write the genotypes of all possible offspring from this cross and use the rules of probability to calculate the chance of each genotype occuring.

12. Three characters (flower color, seed color, and pod shape) are considered in a cross between two pea plants (PpYyIi x ppYyii). What fraction of offspring would be predicted to be homozygous recessive for at least two of the three characters?

13. Non-Dominant Variants
Incomplete dominance: blending appearance between the phenotypes of 2 alleles Ex: snapdragons
Codominance: two alleles that affect the phenotype in separate, distinguishable ways Ex: sickle cell anemia
Multiple alleles: more than 2 possible alleles for a gene Ex: human blood types
Pleiotropy: genes with multiple phenotypic effects. Ex: cystic fibrosis & sickle cell

14. Genetics for Two or More Genes
Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color
Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height

15. Human disorders Can be traced using a family pedigree
Recessive disorders:
Cystic fibrosis
Tay-Sachs
Sickle-cell
Dominant disorders:
Huntington’s
Testing:
Amniocentesis
chorionic villus sampling (CVS)

16. (a) Is a widow’s peak a dominant or recessive trait?
Fig b
1st generation
(grandparents) Ww ww ww Ww
2nd generation
(parents, aunts,
and uncles) Ww ww ww Ww Ww ww
3rd generation
(two sisters) WW ww or
Figure 14.15a Pedigree analysis Ww
Widow’s peak
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?

17. (b) Is an attached earlobe a dominant or recessive trait?
Fig c
1st generation
(grandparents) Ff Ff ff Ff
2nd generation
(parents, aunts,
and uncles) FF or Ff ff ff Ff Ff ff
3rd generation
(two sisters) ff FF or Ff
Figure 14.15b Pedigree analysis
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?