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LM

(b)

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(d)

Phases of Mitosis

A. Interphase B. ProphaseB. C. Metaphase D. Anaphase E. Telophase

• A somatic cell Is a typical body cell Has 46 chromosomes in a human

Is used for sexual reproduction Usually termed eggs and sperm

Review of Terms

• A gametic cell (gamete)

• Haploid – cell or organism that contains one complete set of chromosomes

• Diploid - cell or organism that contains two complete sets of chromosomes

• Autosome Chromosome that is not directly involved in determining the

sex of an organism

A chromosome that determines whether an individual is male or female

Review of Terms

• Sex chromosome

Female

Somaticcells

Male

44

XY

44

XX

Sexual reproduction requires fertilization of an egg by a sperm using a special type of cell division called meiosis.

Meiosis

Meiosis

MEIOSIS I

Sisterchromatidsseparate.

MEIOSIS II

Homologouschromosomesseparate.

INTERPHASE BEFORE MEIOSIS

Sisterchromatids

Duplicated pair ofhomologouschromosomes

Chromosomesduplicate.

Pair of homologouschromosomes indiploid parent cell

Metaphase I

Metaphase II

Recombinant chromosomes

Gametes Recombinant chromosomescombine geneticinformation fromdifferent parents.

Homologous chromatidsexchange correspondingsegments.

Sister chromatids remain joined at theircentromeres.

Prophase Iof meiosis

Duplicated pair ofhomologouschromosomes

Chiasma, site ofcrossing over

Spindlemicrotubule

Crossing overIn crossing over:

•Homologous chromosomes exchange genetic information •Genetic recombination, the production of gene combinations different from those carried by parental chromosomes, occurs

MeiosisHow do we account for genetic variation?

*Independent assortment *Crossing over *Random fertilization

Independent Assortment:

Cross over:

Mitosis and Meiosis

Meiosis I

Abnormal gametes

Gametes

Nondisjunction:Pair of homologouschromosomes failsto separate.

NONDISJUNCTION IN MEIOSIS I

Number of chromosomes

Meiosis II

Nondisjunction:Pair of sisterchromatidsfails to separate.

Abnormal gametes Normal gametes

n n n 1n 1 n – 1n 1

NONDISJUNCTION IN MEIOSIS II

n – 1 n – 1

Errors in Meiosis

Errors in MeiosisDown Syndrome:•Is also called trisomy 21•Is a condition in which an individual has an extra chromosome 21•Affects about one out of every 700 children

Genetics

Genetics is the scientific study of heredity.

Heredity is the transmission of traits from one generation to the next.

Genetics

Gregor Mendel•Worked in the 1860s•Was the first person to analyze patterns of inheritance•Deduced the fundamental principles of genetics

GeneticsRemovedstamensfrom purpleflower.

White

Stamens

Purple

Transferred pollen fromstamens of white flowerto carpel of purpleflower.

Parents(P)

Carpel

Offspring

(F1)

Pollinated carpelmatured into pod.

Planted seedsfrom pod.

Mendel studied garden peas because they:

•Easy to grow

• Come in many readily distinguishable varieties

• Easily manipulated

• Can self-fertilize

Genetics• A character is a heritable

feature that varies among individuals.

• A trait is a variant of a character.

• Each of the characters Mendel studied occurred in two distinct forms.

Genetics

Purple flowers

F1 Generation

White flowers

P Generation(true-breadingparents)

All plants havepurple flowers

F2 Generation

Fertilizationamong F1 plants(F1 F1)

of plants have purple flowers

of plants have white flowers

34

14

Mendel’s hypotheses (to explain his results)

*genes and alleles

1. Alternative versions of genes (alleles) account for variation in inherited characters.

2. For each character, an organism inherits two alleles, one from each parent.

Genetics

3. If two alleles differ, one is dominant, the other recessive

X

Tall Dwarf

P:

DD dd

F1 – all Tall

TallDd

4. The two alleles for each character segregate (separate) during gamete production.

Mendel’s Law of Segregation

Genetics

GeneticsA Punnett Square predicts the results of a genetic cross between individuals of known genotype

D d d

Tall DwarfP: DD ddX

Gamete formation: D

*genotype

*phenotype

*Homozygous

*Heterozygous

D

D

d d

D d D d

D d D d

4/4 are Dd

4/4 are Tall

Dihybrid cross- A genetic cross between two individuals involving two characters

GGWW ggww

Example:

P1

yellow, round

green, wrinkled

XGW GW GW GW

gw

gw

gw

gw

GgWw

Genetics

F1

All yellow, round

F1

All yellow, roundGgWw

F1

All yellow, roundGgWw

X

F2

9/16 yellow, round

3/16 yellow, wrinkled

3/16 green, round

1/16 green, wrinkled

GW Gw gW gw

gW

GW

gw

Gw

9:3:3:1 Phenotypic ratio; Genotypic ratio as follows:

1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw1/16 GGww, 2/16 Ggww

1/16 ggWw, 2/16 ggWw

1/16 ggww

Mendelian inheritance is based on probabilityExample- coin toss

*1/2 chance landing heads

*Each toss is an independent event

*Coin toss, just like the distribution of alleles into gametes

*The rule of multiplication – determines the chance that two or more independent events will occur together

½ x ½ = ¼

B

B

b

Bb

b

Fem

ale

gam

etes B B

B

b b b

Male gametes

Formation of sperm

Bb male

Formation of eggs

Bb female

F2 Genotypes

F1 Genotypes

( )

12

14

12

12

12

14

14

14 1

212

Genetics

Female MaleAttachedFree

Third generation(brother and sister)

Second generation(parents, aunts, and uncles)

First generation(grandparents) Ff Ff

FfFF ff fforFf

ff FForFf

Ffff

Ff ff

Genetics: Pedigrees

Human Disorders

F1 Generation

RR rr

Gametes

P Generation

F2 Generation Sperm

Gametes

Red White

R r

RrPink

R r

R r

R

r

RR Rr

rrRr

Eggs12

12

12

12

12

12

Variations in Mendel’s Laws

In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

Homozygousfor ability to make

LDL receptors

Severe diseaseMild diseaseCell

Normal

LDLreceptor

LDL

Homozygousfor inability to make

LDL receptors

HeterozygousHH Hh hh

GE

NO

TY

PE

PH

EN

OT

YP

E

Hypercholesterolemia•Dangerously high levels of cholesterol in the blood.•Is a human trait that is incompletely dominant.•Heterozygotes have blood cholesterol levels about 2X normal.•Homozygotes have blood cholesterol levels about 5X normal.

Variations in Mendel’s Laws

BloodGroup

(Phenotype)Genotypes Red Blood Cells

O

A

B

AB

ii

IAIB

IBIB

orIBi

IAIA

or IAi

Carbohydrate A

Carbohydrate B

Variations in Mendel’s LawsMultiple Alleles

Variations in Mendel’s Laws– Pleiotropy is the impact of a single gene on more than one

character.

Pleiotropy Multiple traits(e.g., sickle-celldisease)

Singlegene

Variations in Mendel’s Laws

Multiple genes

Polygenicinheritance Single trait

(e.g., skin color)

– Polygenic inheritance is the additive effects of two or more genes on a single phenotype.

Variations in Mendel’s Laws

Variations in Mendel’s Laws

F1 Generation

P Generation

F2 Generation Sperm

AABBCC(very dark)

Eggs

aabbcc(very light)

AaBbCcAaBbCc

18

164 64

66415

6420

6415

646 1

64

18181818181818

18

18

18

18

18

18

18

18

Sex Linkage

*Sex chromosomes*Autosomes

Example: In Drosophila and all mammals

sex chromosomes designated as X and Y

XX=female

XY=male

*1909 Thomas Hunt Morgan

II III IV

XX

XY

or

Any gene located on a sex chromosome is called a sex-linked gene.•Most sex-linked genes are found on the X chromosome.

Sex Linkage