Chapter 14~ Mendel & The Gene Idea

Mendelian genetics

Character (heritable feature, i.e., fur color)

Trait (variant for a character, i.e., brown)

True-bred (all offspring of same variety)

Hybridization (crossing of 2 different true-breds)

P generation (parents)

F 1 generation (first filial generation)

Leading to the Law of Segregation

Alternative versions of genes (alleles) account for variations in inherited characteristics

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).

Mendel’s Law of Segregation

Genetic vocabulary…….

Punnett square: predicts the results of a genetic cross between individuals of known genotype

Homozygous: pair of identical alleles for a character

Heterozygous: two different alleles for a gene

Phenotype: an organism’s traits

Genotype: an organism’s genetic makeup

Testcross: breeding of a recessive homozygote X dominate phenotype (but unknown genotype)

The Law of Independent Assortment

Law of Segregation involves 1 character. What about 2 (or more) characters?

Monohybrid cross vs. dihybrid cross

The two pairs of alleles segregate independently of each other.

Mendel’s Law of Independent Assortment

Non-single gene genetics, I

Incomplete dominance : appearance between the phenotypes of the 2 parents. Ex: snapdragons, sickle cell anemia

Codominance: two alleles affect the phenotype in separate, distinguishable ways. Ex: Tay-Sachs disease, human blood types A and B

Multiple alleles: more than 2 possible alleles for a gene. Ex: human blood types

Effects of antiserum on blood types

Probability: An overview

Probability: the chance of an event occurring. Expressed as a fraction or ratio

Rule of multiplication: The chance that different events all occur simultaneously is the product of the probabilities of each event occurring independently.

Probability of getting three heads:

½ X ½ X ½ = 1/8

Probability 2

Rule of addition: The chance of an event happening that can occur in two or more different ways is the sum of the probabilities of the different events.

Chance of getting one head and one tails when you toss two coins:

¼ + ¼ = ½

Probability 3

For the cross: PpYyRr X Ppyyrr

What is the probability of getting a individual that expresses the recessive form for at least two traits?

P = purple flowers p = white flowers

Y = yellow seeds y = green seeds

R = round seeds r = wrinkled seeds

Probability 4

ppyyRr ¼ X ½ X ½ = 1/16

ppYyrr ¼ X ½ X ½ = 1/16

Ppyyrr ½ X ½ X ½ = 2/16

PPyyrr ¼ X ½ X ½ = 1/16

ppyyrr ¼ X ½ X ½ = 1/16

Chance of at least 2 recessive = 6/16 or 3/8

Pleiotropy

Epistasis

Polygenic
Inheritance

Non-single gene genetics, II

Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia

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

Multifactorial Traits

Pedigrees and Traits

Sample Alkaptonuria Pedigree

Human disorders

The family pedigree

Recessive disorders: •Cystic fibrosis •Tay-Sachs •Sickle-cell

Dominant disorders: • Huntington’s

Testing: •amniocentesis •chorionic villus sampling (CVS)

Female human Karyotype

Male human karyotype