Genetics

Chapter 11

Heredity

• Passing of traits from parent to offspring

Genetics

• The study of heredity

• Gregor Mendel- Austrian monk known as the “father of genetics”

Mendel’s Four Hypotheses

1. For each inherited trait, an individual has a copy of that gene from each parent.

2. There are alternate versions of genes, called alleles. (ex: freckles or no freckles)

Hypothesis continued

3. When 2 different alleles occur together, one of them may be completely expressed (dominant), while the other may not be observed (recessive).

4. Alleles separate independently, so that each gamete only carries 1 allele for that trait.

Monohybrid cross

• Mendel began with 1 trait (monohybrid) crosses

• He bred pea plants to produce several generations

P- the parent generation

F1 - the first filial generation

F2 - second filial generation

• Mendel concluded that each parent has two separate “factors” for a particular trait

• “Factors” are now called genes.

Mendel’s Laws of Heredity

1. Law of Segregation- two alleles separate when gametes are formed

2. Law of Independent Assortment- alleles of different genes separate independently of one another during gamete formation

fertilization produces heterozygous offspring

meiosis II

meiosis I

(chromosomes duplicated

before meiosis)

Homozygous dominant parent

Homozygous recessive parent

(gametes) (gametes)

Fig. 11-5, p.172

Mendel’s Law of

Segregation

Nucleus of a diploid (2n)reproductive cell with two pairs of homologouschromosomes

Possible alignmentsof the two homologouschromosomes duringmetaphase I of meiosis

The resulting alignments at metaphase II

Allelic combinationspossible in gametes

1/4 AB 1/4 ab 1/4 Ab 1/4 aB

Fig. 11-8, p.174

Independent Assortment

Some modern genetic terms

• Alleles are represented by letters

– Dominant allele is a capital letter

– Recessive allele is a lower case letter

• Homozygous- identical alleles for a specific trait (BB, FF, rr, tt)

• Heterozygous- alleles are different for a specific trait (Bb, Ff, Rr, Tt) (aka “hybrid”)

• Genotype- the set of alleles an individual inherits for a trait (i.e.- Rr, Ww, ff)

• Phenotype- the physical expression of a trait (i.e. blue eyes, freckles, dimples)

Punnett Square

• Predicts possible outcomes of traits

• Shows all possible outcomes of a genetic cross

• Probability (likelihood that an event will occur) of a genetic outcome can be predicted

• Ex: cross two heterozygous individuals Aa x Aa

predict ratios of

3:1 for phenotype and

1:2:1 for genotype

Practice

• Curly hair is dominant over straight hair. A man with straight hair and a woman who is heterozygous for curly hair have a child. What is the probability that this child will have straight hair?

• Incomplete Dominance- when an individual shows a combination of the inherited alleles.

Ex: red snapdragon x white snapdragon will produce a pink snapdragon

straight hair x curly hair = wavy hair

• Codominance- 2 dominant alleles are expressed at the same time

Ex: Roan horses show both red and white hairs in equal numbers

Patterns of heredity can be very complex

• Sex-linked trait- a trait whose alleles is located on the X chromosome (most are recessive)

• Polygenic trait- trait where several genes influence the outcome (eye color, hair color, skin color, height, weight)

• Multiple Alleles- genes with 3 or more alleles

- Blood type is an example of multiple alleles because we have A, B and O alleles and 4 possible blood type outcomes

(A, B, AB and O)

Fig. 11-10a, p.176

Range of genotypes:

IAIA IBIB

IAi IAIB IBi ii

BloodTypes:

or

A AB B O

or

ABO Blood Type

Diploid

• Number of chromosomes found in the body or somatic cells of an organism

• “2n”

Haploid

• Number of chromosomes found in the gametes of an organism

• “n”

Meiosis

• The type of cell division that produces gametes

• Gametes = sex cells– female gamete = egg or ovum (plural = ova)– male gamete = sperm

Meiosis

Formation of sperm and egg