Introduction to Genetics

Genetics = the scientific study of heredity

Check Your Understanding

Check Your Understanding

Chromosomes– Tightly coiled

DNA where your genes are carried.

Genes– Sections of DNA

that control your traits.

The Work of Gregor Mendel

Austrian monk.Studied heredity.

– Heredity: the passing on of characteristics from parents to offspring.

– Characteristics that are inherited are called traits.

First person to successfully predict how traits are transferred from generation to generation.Used garden peas in his experiments.

The Father of Genetics

Why pea plants?– Reproduce sexually, which means

that they produce male and female sex cells, called gametes.

– In a process called fertilization, the male gamete unites with the female gamete.

– The resulting fertilized cell, called a zygote, then develops into a seed.

He took pollen from a male plant and dusted it onto a female plant.

– Parental generation (p) = the original pair of plants

Mendel’s Experiment

Female part

Transfer pollen

Pollen grains

Maleparts

Cross-Pollination

• Offspring

• (f1) = first filial generation

• (f2) = second filial generation

His first experiments are called monohybrid crosses because they only deal with ONE single trait (height, color)

• mono means “one”

Each organism has two factors that control each of its traits. These factors are genes and that they are located on chromosomes.Genes exist in different forms called alleles.

Conclusions

Tall pea plant

Short pea plant

All tall pea plants

3 tall: 1 short

P1

F1

F2

Alleles

The different forms a gene can take (usually represented by letters).2 Types of Alleles:

1. Dominant – allele that is always displayed; ex “A”.

2. Recessive – allele only displayed when paired w/ another recessive allele; ex “a”.

Mendel called the observed trait dominant and the trait that disappeared recessive.Mendel concluded that the allele for tall plants is dominant to the allele for short plants.

T T

T

T

t t

t

t

Tall plant Short plant

All tall plants

F1

Genotype– Letter representation of the genetic

make-up of an individual (AA, Aa, aa).

Phenotype– An observable physical characteristic

of an organism. (Tall/short, blue/brown, curled/straight).

F1

Tall plant Tall plant

TTT

T t T t

t T t tt3

Tall Tall Short

1

Tall

F2

Law of Segregation• The way an organism

looks and behaves is called its phenotype. Ex. Tall, yellow

• The allele combination an organism contains is known as its genotype. Ex. TT, Tt

• An organism’s genotype can’t always be determined by its phenotype.

Tt xTt cross

An organism is homozygous for a trait if its two alleles for the trait are the same. (True-breeding)

– Exp. TT or tt

An organism is heterozygous for a trait if its two alleles for the trait differ from each other. (Hybrid)

– Exp. Tt

Genetics & Probability

Probability = the likelihood that a particular event will occur:

– Ex. Coin flipping: 1/2 probability that coin will flip head/tail

– If you flip the coin 3 times what's the probability of flipping 3 heads?

1/2 x 1/2 x 1/2 = 1/8

– *Past outcomes do not affect future ones!!*

The principles of probability can be used to predict the outcomes of genetic crosses.

– What is the probability of parents having two male offspring in a row? (1/2 x 1/2=1/4)

Punnett Squares

Are used to predict and compare the genetic variations that will result from a cross.

– The types of gametes go on the top and left sides of the square.

– The possible gene combinations appear in the four boxes.

If you know the genotypes of the parents, you can use a Punnett square to predict the

possible genotypes of their offspring.

T t

T

t

T T T t

T t t t

You try this one…

A A

A

a

Check Your Understanding

Question 1

The passing on of characteristics from parents to offspring is __________.

D. allelic frequency

C. pollination

B. heredity

A. genetics

The answer is B. Genetics is the branch of biology that studies heredity.

Check Your Understanding

Question 2

What are traits?

AnswerTraits are characteristics that are inherited. Height, hair color and eye color are examples of traits in humans.

Check Your Understanding

Question 3

Gametes are __________.

D. fertilized cells that develop into adult organisms

C. both male and female sex cells

B. female sex cells

A. male sex cells

The answer is C.

Organisms that reproduce sexually produce male and female sex cells, called gametes.

Check Your Understanding

Question 4

Which of the following genotypes represents a animal that is homozygous dominant for a trait?

a. KK

b. Kk

c. kk

Check Your Understanding

Question 5

Which of the following genotypes represents a plant that is homozygous recessive for height?

c. tt

b. Tt

a. TT

Exceptions to Mendel

Genetics: Part 2

Mendel’s Dihybrid Crosses

Dihybrid Cross

round yellow x wrinkled green

Round yellow

Wrinkled green

All round yellow

Round yellow

Round green

Wrinkled yellow

Wrinkled green

9 3 3 1

P1

F1

F2

Law of Independent Assortment

• He found they appeared in a definite ratio of phenotypes 9:3:3:1• 9 round yellow: 3 round green: 3 wrinkled

yellow: 1 wrinkled green.

• He concluded that the seed shape and seed color—are inherited independently of each other.

• This conclusion is known as the law of independent assortment.

Dihybrid Crosses

A Punnett square for a

dihybrid cross will need to be four boxes on each side for a

total of 16 boxes.

Gametes from RrYy parentRY Ry rY ry

Gam

ete

s fr

om

RrY

y

pare

nt

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

Not all genes show simple patterns of dominant and recessive alleles.

– Because the majority of traits are controlled by more than one gene/alleles.

Exceptions to Mendel

Incomplete Dominance

Cases in which one allele is not completely dominant over another. Neither allele is dominant.

– Ex. Red flower x White flower = Pink flower

Codominance

Cases in which both alleles are expressed.– Ex. White cow x Black cow = Speckled

(white and black) cow.

Cow-1.au

Multiple Alleles

When a gene has more than two alleles.•Ex. Mice coat color is determined by one gene with four different alleles. Three different colors result from the combinations of these alleles.

Blood TypesMultiple Alleles & Codominance

In humans, there are four blood types (phenotypes): A, B, AB, and O Blood type is controlled by three alleles. A, B, O O is recessive, two O alleles must be present for the person to have type O blood A and B are codominant. If a person receives an A allele and a B allele, their blood type is type AB Crosses involving blood type often use an I to denote the alleles. See chart.

Type O's are automatically OO and type AB is automatically AB. Type A and B can be homozygous or heterozygous.

Polygenic Traits

Traits controlled by two or more genes.– Ex. At least three genes are involved in

making the reddish-brown pigment in the eyes of fruit flies. Different combinations of alleles for these genes yield different eye colors.

Sex Linked Traits

Some genes are located on the X chromosome. Females receive two alleles (XX) for these genes, but males only receive one (y).

When doing a Punnett Square, use large X's and Y's to denote male and female, use superscript letters to designate the alleles.

– Ex. hemophilia (bleeding) and color blindness are recessive sex-linked traits (XcXc or XcY).

If the parent is a male, the genotype is automatically known. A colorblind male has to be b (recessive), since he only has one allele and colorblindness is recessive. A normal male must then be B (dominant).

Females can be heterozygous for the colorblindness trait - they are called carriers. A female can be XBXB - normal, XBXb - carrier, or XbXb – colorblind.

The following shows a cross between a normal man and a woman who is a carrier.

What…

MORE practice problems?

Homer_imeanwoohoo.mp3

Check Your Understanding

Question 1In a certain cactus, prickly spines can be two pronged or one pronged. If a true breeding (homozygous) one-pronged cactus is crossed with a true breeding two-pronged cactus, the F1 generation has a mixture of spines, some are two-pronged, some are one-pronged.

codominance

Is this an example of codominance or incomplete dominance?

Check Your Understanding

Question 2

Parents Blood Types

Mr. Hartzel O

Mrs. Hartzel A

Mr. Simon AB

Mrs. Simon AB

Mr. Peach O

Mrs. Peach O

Baby Blood type

Jennifer O

Rebecca A

Holly B

A nurse at a hospital removed the wrist tags of three babies in the maternity ward. She needs to figure out which baby belongs to which parents, so she checks their blood types. Using the chart below, match the baby to its correct parents.