Mendel & The Gene Idea

Chapter 14

Mendelian Genetics

■ ■ ■ ■ ■ ■ ■ Character  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 F 

1

generation first filial generation; filial = “son”; product of P generation F 2  generation product of crossing two from the F 1 generation

■ ■ ■ ■ ■ ■

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)

1.

2.

3.

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

■ ■ ■

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

Practice

1.

A pea plant heterozygous for inflated pods ( (ii). Draw a punnett square for this cross and I i) is crossed with a plant homozygous for constricted pods determine the genotypic and phenotypic ratios.

2.

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 (7 th ed)or 265 (8 th ed)

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

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

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

Practice

1.

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?

2.

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.

3.

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

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

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

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)

Fig. 14-15b 1st generation (grandparents)

Ww ww ww Ww

2nd generation (parents, aunts, and uncles) 3rd generation (two sisters)

Ww ww ww Ww Ww ww WW

or

Ww ww

Widow ’ s peak (a) Is a widow ’ s peak a dominant or recessive trait?

No widow ’ s peak

Fig. 14-15c 1st generation (grandparents)

Ff Ff ff Ff

2nd generation (parents, aunts, and uncles) 3rd generation (two sisters)

FF

or

Ff ff ff Ff ff Ff FF

or

Ff ff

Attached earlobe (b) Is an attached earlobe a dominant or recessive trait?

Free earlobe