Genetics

The Study of Heredity

Gregor Mendel

• Discovered many of the principles of modern genetics

• Mendel studied the inheritance of traits using pea plants

• Principles of basic inheritance are called Mendelian genetics

Genes• Genes are segments of

chromosomes that determine the traits of an organism (ex. Eye color)

• There may be several hundred to several thousand genes on a chromosome

• We inherit 50% of our genes from our mothers and 50% from our fathers

50% of your genesfrom dad

50% of your genesfrom mom

Alleles

• Every gene comes in different variations called alleles

• Most genes have two different alleles while some have more than two

• One person can have no more than two different alleles for the same gene (we get one from each parent)

Three people with different alleles for a gene involved in eye color

Dominant vs. Recessive Alleles

• Alleles interact to produce traits (eye color, shape of skeleton, blood type, etc.)

• Dominant alleles “overpower” recessive alleles (see diagram)

Browneyes

Browneyes

Blueeyes

Human Karyotype (picture of all 46 chromosomes)

Each pair is called a homologous pair

Homologous Chromosomes

• Chromosomes that are very similar (size, shape, same type of genes) except that they come from two different individuals (see diagram)

Mother Father

A pair of homologous chromosomesfrom one individual

Monohybrid Crosses

Using probability to determine the likely inheritance of a trait

ALBINISMLack of pigment in skin, hair, and eyes

Example problem

A= normal pigmentation a= albino

The allele for normal skin pigment (A) is dominant over the allele for albinism (a). If a heterozygous normal pigmented man (Aa) marries a homozygous albino woman (aa), will they have any albino children?

Punnett Square

A

a

a

a

Aa

aaAa

aa

Possible gene combinations (genotypes) of children

DAD’s possible gametes

MOM’s possible gametes

What percentage of their children have normal skin

pigment?

The allele for normal pigment (A) is dominant over the allele for albinism (a).

WHY?

Genotype vs. Phenotype

• Genotype is the genetic make-up of an organism (its genes)

• Phenotype refers to the actual physical traits an organism has as a result of its genes

• The genotype determines the phenotype (see picture)

The genes of the fly give it itsunique characteristics

Heterozygous & Homozygous

• Homozygous - having the same alleles for a given trait

• Heterozygous - having different alleles for a trait

Meiosis

• Formation of gametes (sperm and egg)

• Happens in testes (males) and ovaries (females)

• Reduces the chromosome number by half

• Meiosis produces 4 genetically different cells from the original cell

Why are the cells produced in meiosis different?

• When gametes are made, they receive only one chromosome from each homologous pair, making haploid cells (have half the number of chromosomes as the original cell)

• Chromosomes are distributed to gametes randomly resulting in different combinations of chromosomes (called independent assortment)

Independent assortment - random distribution of chromosomes into the gametesduring meiosis

Why are the cells produced in meiosis different? (continued)

• Chromosome shuffling (independent assortment)

• Crossing over - homologous chromosomes exchange genetic information forming new combinations of genes (see diagram)

• Mutations - a change in the DNA (genes) of an organism

Crossing over during prophase Iof meiosis

Mitosis vs. Meiosis

One cell division Results in 2 diploid

cells (full set of chromosomes

Daughter cells are identical

Happens in body cells (somatic cells)

For growth and repair

• Two cell divisions• Results in 4 haploid

cells (half # of chromosomes)

• Daughter cells are different

• Happens in sex cells (gametes)

• For maintaining the chromosome # of the species

Mendel’s Law of Segregation

• Alleles for the same trait separate from each other during the formation of gametes

(see Punnett square diagram)

Mendel’s Law of Independent Assortment

• Alleles for different traits are randomlydistributed to gametes (unless they are very closely linked)

Predicting Genetic Crosses with Punnett

Squares• Monohybrid crosses

(shows segregation)• Dihybrid crosses (shows segregation

and independent assortment)

Example of a dihybrid cross

Dihybrid cross (example) A heterozygous guinea pig for black, rough fur

mates with another heterozygous guinea pig for black, rough fur. What will the genotypes and phenotypes of their offspring be given the information below:

B = black furb = white furR = rough furr = smooth fur

Answer to Problem

• Black, rough fur (B__R__)

• Black, smooth fur (B__rr)

• White, rough fur (bbR__)

• White, smooth fur (bbrr)

9

3

3

1

A 9:3:3:1 ratio occurs when two heterozygous parentshave offspring in a dihybrid cross

Human Inheritance

Chapter 12

Patterns of Inheritance

• Complete dominance

• Incomplete dominance

• Codominance

• Polygenic Inheritance

• Sex-linked inheritance

Incomplete Dominance

Neither allele of a pair is dominant over the other

A blend of both traits results

Genotypes and phenotypes of flowers showing incomplete dominance for petal color (R= red and W=white)

Codominance Both alleles show

dominance causing both traits to be

expressed in the phenotype of the organism

Examples: 1. Coat color in animals

2. Blood type in humans Example of black roan coatshowing codominance

Blood typeBlood type

3 different alleles for blood type exist:

IA and IB are codominant to each other

IA and IB are both dominant to i

3 different alleles for blood type exist:

IA and IB are codominant to each other

IA and IB are both dominant to i

Genotypes of the 4 blood types:

IAIA or IAi = type A IBIB or IBi = type B IAIB = type AB ii = type O

Genotypes of the 4 blood types:

IAIA or IAi = type A IBIB or IBi = type B IAIB = type AB ii = type O

Possible outcomes of children of dad with type AB blood and mom with type O blood

Polygenic Traits• Traits that are controlled by the combined

action of many genes

• Show a diverse range of phenotypes

• Most traits are polygenic

• Ex. Skin color, hair color, eye color, height, risk for heart disease, etc.

Sex-linked traits Any trait that is

carried on the X or Y chromosome

If allele is on the X chromosome, females will have two copies (XX) but males only one (XY)

Example: Hemophilia (blood does not clot properly)

Independent assortment ofsex chromosomes

Symbolizing Sex-Linked Traits

Hemophilia is a sex-linked recessive trait that is carriedon the X chromosome.

XH = normal Xh = hemophilia

* Y chromosome has no gene for hemophilia

XH XH

XH Xh

Xh Xh

= normal woman

= carrier (does not have disorder but carries the allele)

= hemophiliac female

XH

Xh

YY

= normal man

= hemophiliac male

Possible genotypes and phenotypes