GENETICS

Gregor Mendel• Genetics is the study

of heredity.

• Gregor Mendel (1860’s) discovered the fundamental principles of genetics by breeding garden peas.

Mendel cross-pollinated pea

plants

• He cut away the male parts of one flower, then dusted it with pollen from another

What did Mendel conclude?

• He concluded that factors are passed from one generation to the next.

The Principle of Dominance

• Alleles1. Alternative forms of genes.2. Units that determine heritable traits.

Paternal Maternal

eye color locusB = brown eyes

eye color locusb = blue eyes

The Principle of The Principle of DominanceDominance – some – some alleles are alleles are dominant and other dominant and other are recessive.are recessive.

Dominant and Recessive alleles

Dominant alleles – upper-case a. homozygous dominant

(BB – Brown eyes)

Recessive alleles – lower casea. homozygous recessive

(bb – blue eyes)b. Heterozygous (Bb – Brown eyes)

Phenotype vs. Genotype

• Outward appearance• Physical characteristics

• Examples:1.Brown eyes

2.blue eyes

• Arrangement of genes that produces the phenotype

• Example:1. TT, Tt2. tt

SegregationAlleles separate during meiosis

Recessive traits show up about 1/4th of the time.

• Because there is only a 25% chance that two recessive alleles will be paired together.

Punnett square

• A Punnett square is used to show the possible combinations of gametes.

• Monohybrid Cross

Monohybrid Cross

• Example:Cross between two heterozygotes for brown eyes (Bb)

BB = brown eyesBb = brown eyesbb = blue eyes

B

b

B b

Bb x Bb

malegametes

female gametes

Monohybrid Cross

BB

Bb

Bb

bb

B

b

B b

Bb x Bb

1/4 = BB - brown eyed1/2 = Bb - brown eyed1/4 = bb - blue eyed

1:2:1 genotype 3:1 phenotype

Dihybrid Cross

Dihybrid Cross

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

Dihybrid Cross

RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

Dihybrid Cross

• Example:cross between round and yellow heterozygous pea seeds.

R = roundr = wrinkledY = yellowy = green

RY Ry rY ry RY Ry rY ry x RY Ry rY ryRY Ry rY ry possible gametes produced

RrYyRrYy x RrYyRrYy

Independent Assortment• Chromosomes separate

independently of each other

Bb

diploid (2n)

B

b

meiosis I

B

B

b

b

sperm

haploid (n)

meiosis II

Bb

Ff

B

F

B

f

b

f

B

F

Bb

Ff

Bb

Ff

This means all gametes will be different!

Independent Assortment• Genes for different traits can segregate

independently during the formation of gametes without influencing each other

• Question: How many gametes will be produced for the following allele arrangements?

• Remember: 2n (n = # of heterozygotes)

1. RrYy2. AaBbCCDd3. MmNnOoPPQQRrssTtQq

Answer:

1. RrYy: 21. RrYy: 2nn = 2 = 222 = = 4 gametes4 gametes

RY Ry rY ryRY Ry rY ry

2. AaBbCCDd: 22. AaBbCCDd: 2nn = 2 = 233 = = 8 gametes8 gametes

ABCD ABCd AbCD AbCdABCD ABCd AbCD AbCd

aBCD aBCd abCD abCD aBCD aBCd abCD abCD

3. MmNnOoPPQQRrssTtQq: 23. MmNnOoPPQQRrssTtQq: 2nn = 2 = 266 = = 64 gametes64 gametes

Incomplete Dominance

• One allele is not completely dominant over another

Rr

Rr

Rr

Rr

r

r

R R

All Rr = pink(heterozygous pink)

produces theFF11 generation generation

Codominance

• Both alleles are expressed• Example: blood

1. type A = IAIA or IAi

2. type B = IBIB or IBi

3. type AB = IAIB

4. type O = ii

Black cow + white cow = spotted cow

Which shows more genetic variation. (more combos?)

• Male CC with female Cc • Male cc with female Cc

C

c

c c

C

c

C C

Genetic Engineering

Gene Therapy

Is cloning a possibility?

• Is it right to use cloning to create an entirely new human being?

• Is it ethical to create an embryonic copy of John Doe to supply cells to keep John alive?

• Does a multicellular ball of tissue -- an embryo -- have the same rights and status as a human being?

How does Mendel’s principles apply to organisms

• The basic principals can be applied to humans as well as any other living organism.

• Consider thisConsider thisPunnett square:Punnett square:

• Many traits are controlled by multiple alleles Many traits are controlled by multiple alleles or multiple genes. or multiple genes. – Multiple alleles (more than 2 choices)Multiple alleles (more than 2 choices)– Polygenic (multiple genes control a single trait)Polygenic (multiple genes control a single trait)

Multiple alleles:Multiple alleles:

• the case where three or more alleles of the the case where three or more alleles of the same gene exist. Remember, an organism same gene exist. Remember, an organism will have only two of these alleles (one from will have only two of these alleles (one from mom and one from dad).mom and one from dad).

– Examples: Coat color in rabbits, blood type in Examples: Coat color in rabbits, blood type in humanshumans

Multiple alleles:Multiple alleles:

Polygenic traits:Polygenic traits:

• traits that are determined by alleles from traits that are determined by alleles from more than one gene; these traits usually more than one gene; these traits usually have a range of phenotypeshave a range of phenotypes

– Examples: skin color in humans, height in Examples: skin color in humans, height in humanshumans

Mapping GenesMapping Genes

• ItIt’’s easy to imagine that genes on different s easy to imagine that genes on different chromosomes assort independently, but what about chromosomes assort independently, but what about genes that occur on the same chromosome? Dongenes that occur on the same chromosome? Don’’t t they always appear together?they always appear together?

• Not always due to crossing over. Genes that occur Not always due to crossing over. Genes that occur together on a chromosome will be separated when together on a chromosome will be separated when homologous chromosomes exchange genes. homologous chromosomes exchange genes.

• The frequency of genes occurring together can help The frequency of genes occurring together can help us generate a gene map. us generate a gene map.

• The more often two genes occur together, the The more often two genes occur together, the closer they are to each other on the closer they are to each other on the chromosome. chromosome. – If the genes are never separated by crossing over, If the genes are never separated by crossing over,

they always occur together. All offspring will look they always occur together. All offspring will look like one of the parents (in reference to the genes like one of the parents (in reference to the genes in question). in question).

– If half of the offspring are parental and half are If half of the offspring are parental and half are recombinations of the parents (in reference to the recombinations of the parents (in reference to the genes in question), then they are said to be genes in question), then they are said to be independent. This means they are either on independent. This means they are either on separate chromosomes or they are almost always separate chromosomes or they are almost always separated during meiosis.separated during meiosis.

– You will learn to calculate distances and create a You will learn to calculate distances and create a map in AP Bio, or in collegemap in AP Bio, or in college

Human chromosomesHuman chromosomes

2.2. There are two types of chromosomes.There are two types of chromosomes.a.a. Autosomes: Of the 46 chromosomes, 44 of them (22 Autosomes: Of the 46 chromosomes, 44 of them (22

pairs of chromosomes) are called autosomes (non-sex pairs of chromosomes) are called autosomes (non-sex chromosomes).chromosomes).

b.b. Sex chromosomes: The last two chromosomes are called Sex chromosomes: The last two chromosomes are called the sex chromosomes because they determine the sex of the sex chromosomes because they determine the sex of the person. Females have two X chromosomes (XX) and the person. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).males have one X and one Y chromosome (XY).

3.3. GametesGametesa.a. All gametes are haploid. In humans, that means All gametes are haploid. In humans, that means

each egg cell and each sperm cell has 1 copy of each egg cell and each sperm cell has 1 copy of each chromosome for a total of 23 each chromosome for a total of 23 chromosomes. chromosomes.

i.i. Egg cells: All human egg cells carry 23 chromosomes, Egg cells: All human egg cells carry 23 chromosomes, one of which is a single X chromosome. This is one of which is a single X chromosome. This is written as 23, X. written as 23, X.

ii.ii. Sperm cells: In males, there are two types of sperm Sperm cells: In males, there are two types of sperm cells- one carries an X chromosome (23, X) and one cells- one carries an X chromosome (23, X) and one carries a Y chromosome (23, Y). carries a Y chromosome (23, Y).

iii.iii. When a sperm and egg When a sperm and egg cell combine, half of the cell combine, half of the time the fertilized eggs time the fertilized eggs (also called zygotes) are (also called zygotes) are female (46, XX) and half female (46, XX) and half of the time they are of the time they are male (46, XY).male (46, XY).

X X

X

Y

XX XX

XY XY

female female

male male

sperm

eggs

• Sex Linked traits: traits that are determined by alleles that are found on the X or Y chromosome.– The Y chromosome is

shorter and does not carry all the same alleles as the X chromosome.

– Females are XX and males are XY. – Females can be homozygous or heterozygous for

a trait carried on the X chromosome, but males (having only one X chromosome) are hemizygous.

– If they inherit a defective gene from the parent, then they will exhibit the trait because they cannot inherit a second gene to mask it.

– Conversely, a healthy male cannot be “hiding” a bad recessive allele because they only have one X chromosome.

Example of a sex-linked Punnett square: Example of a sex-linked Punnett square: – XXBBXXbb (heterozygous female with normal vision) crossed to X (heterozygous female with normal vision) crossed to XBBY Y

(hemizygous male with normal vision)(hemizygous male with normal vision)

XBY

XBY

XbY

XB XB

XB Xb

YXB

XB

Xb

XB Xb

Genetics and the EnvironmentGenetics and the Environment

• Characteristics are determined by both genes Characteristics are determined by both genes and the environment. and the environment.

• External: While genes will influence the height External: While genes will influence the height of a plant, the amount of water, sun, and of a plant, the amount of water, sun, and other climate conditions will also affect the other climate conditions will also affect the height.height.

Genetics and the EnvironmentGenetics and the Environment

• Internal: There are recent findings that Internal: There are recent findings that proteins involved with DNA can turn genes on proteins involved with DNA can turn genes on or off based on environmental factors. or off based on environmental factors. – Certain chemical exposure can turn genes on or Certain chemical exposure can turn genes on or

off (make the traits show up or not) for off (make the traits show up or not) for generations after exposure, but there are no generations after exposure, but there are no changes to the DNA (no mutations). changes to the DNA (no mutations).

– This new understanding of how genes are This new understanding of how genes are expressed is called epigenetics.expressed is called epigenetics.