Chapter 10:

Chapter 10:

Patterns of Inheritance

Thinking Time!!!!!!!!!

• Why is it, in some cases, that two brown-eyed parents have all brown-eyed children, and in other cases two brown-eyed parents have some brown-eyed children and some blue-eyed children?

• What do you think would be the result of a cross between a red flower and a white flower?

Cracking the code of life

1. The Blending Hypothesis:– Predicts that offspring appearances will be blends of the

phenotypes of their parents.

2. The Particulate Hypothesis (Mendel): – One copy of each factor is inherited from each parent.– The factors remain intact and do not physically blend

together.

+ red white all pink

+ red white

+red white

Genetic Vocabulary:

• Trait: Variation of a particular character. – Ex: eye color is a character,

• Hybrids: The offspring of 2 different true-breeding species.

• Monohybrid cross: a pairing in which the parents differ in only one (mono) character. – Ex: Flower color

traits

while

+ mule

P generation: Parent plants

F1 generation: The hybrid offspring

F2 generation: When F1 plants self-fertilize or fertilize each other, their offspring makes up the F2 generation.

Terms need to know:

• Allele: The different forms of genes. – Organisms have two alleles for each trait, one from each

parent.

• Dominant: appear to hide other traits.– capital letters

• Recessive: can be hidden in one generation and then appear in the next.– lower-caser letters

• Phenotype - the way an organisms’ genes express themselves (physical type, description)– either short, tall, purple, white, & etc

• Genotype - the gene type of an organism (genes, letters)– TT, Tt, tt

ABCDEFGH

abcdefgh

Homozygous vs. Heterozygous

In pea plants,

• Homozygous - 2 alleles are the same.– homozygous dominant

• 2 dominant genes : PP– homozygous recessive

• 2 recessive genes : pp• Heterozygous – 2 alleles are different.

– 1 dominant gene and 1 recessive gene: Pp.

is dominant over

Gregor Mendel

• Often called the "Father of Genetics”• Performed many experiments with

plants, mostly garden peas.• His 1st step: identify true-breeding

plant.• 2nd step: He crossed true-breeding

plants that had two distinct traits.

?

Mendel’s First Experiment:

¾ of plants are purple¼ of plants are white

Mendel’s Results:• He examined the inheritance patterns of 7 different

pea-plant characters. • For each character, one of the two parent traits

disappeared in the F1, but reappeared in ¼ of the F2 generation.

The Testcross:

• It is not possible to predict the genotype of an organism with a dominant phenotype.

• It could be homozygous dominant PP or heterozygous Pp.

• A test cross can determine the identity of the unknown allele.

Writing Genotypes from phenotypes:

For Height: tall (T) is dominant and short (t) is recessive.

• 1. homozygous dominant for height– Answer: TT

• 2. heterozygous for height– Answer: Tt

• 3. homozygous tall– Answer: TT

• 4. recessive for height– Answer: tt

• 5. heterozygous tall– Answer: Tt

Writing phenotypes from genotypes:For Height:

tall (T) is dominant and short (t) is recessive.For Flower color:

red (R) is dominant and white (r) is recessive.• 6. TT

– Answer: homozygous dominant, tall• 7. tt

– Answer: homozygous recessive, short• 8. Tt

– Answer: heterozygous, tall• 9. Rr

– Answer: heterozygous, red• 10. rr

– Answer: homozygous recessive, white

Activity Time:The Coin Activity

• When tossing one penny, what is the probability of getting one head?

• When tossing one penny, what is the probability of getting one tail?

When tossing two pennies, what is the probability of getting one head and one tail?

Punnett Squares

• It shows the possible outcomes for the Phenotypes of the individual.  

Predicting Genotypes & Phenotypes

• In pea plants,

• Cross: homozygous tall X homozygous short

Tall Short

is dominant

over

Step 1:

• Write down the genotypes of each parent – ABCDEFGH dominant traits– abcdefgh recessive traits

• Homozygous tall:

• Homozygous short:

TT

tt

Step 2:

• Draw a Punnett square• Write the gametes of one parent across the top

and the other parent along the side

T T

t

t

Step 3:

• Fill in each box of the Punnett square– Capital letter goes 1st

T T

t

t TT

TT

tt

tt

Step 4:

• List the possible genotypes and phenotypes of the offspring.

• Possible genotypes:

• Possible phenotypes:

Tt

Tall

T T

t

t

Tt

Tt Tt

Parent 1:

Parent 2:

Tt

Results

Results

• Each of the offspring has a ____ /4 or _____% chance of showing ______ genotype.

• Since T (tall) is dominant over t (short). Therefore, each of the offspring has a ____/4 or _____% chance of being __________.

Tt

1004

1004

tall

Key: brown eyes (B) is dominant, blue eyes (b) is recessive

Homozygous dominant X Heterozygous • Genotypes: BB & Bb

B B

B

b

BB BB

Bb Bb

Practice Time:

is dominant over

Questions:1. Phenotype of offspring?2. Genotype of offspring?3. Phenotypic ratio:4. Genotypic ratio:

all brown

BB & Bb

2 BB: 2Bb

4 brown: 0 blue

Your Turn

Questions:1. Phenotype of offspring?2. Genotype of offspring?3. Phenotypic ratio:4. Genotypic ratio:

is dominant over

In humans,

heterozygous brown X heterozygous brown

brown & blueBB, Bb & bb

3 Brown: 1 blue

1 BB: 2 Bb: 1 bb

Dihybrid Cross

• In one dihybrid cross experiment, Mendel studied the inheritance of seed color and seed shape.

is dominant over

Yellow (Y) green (y)

is dominant over

Round (R) wrinkled (r)

Dihybrid cross:

• Mendel crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that has green, wrinkled seeds (yyrr).

• The first parent could only produce RY gametes. The other could only produce ry gametes.

• The union of these gametes results in RrYy.

Dihybrid Cross:

• Four classes of gametes (YR, Yr, yR, and yr) are produced in equal amounts.

• Which results in 16 equally probable ways in which the alleles can combine in the F2 generation.

• Phenotypic ratio - 9:3:3:1

Intermediate Inheritance:

• In Mendel’s pea crosses, the F1 offspring always looked like the dominant homozygous parent. – Because the recessive phenotype required two

recessive alleles.

– But, for some characters of organisms, neither allele is dominant intermediate inheritance.

YY yellow

Yy yellow

yy green

Dominant

• Heterozygous F1 generation hybrids do not show the phenotype of the dominant allele or the recessive allele but somewhere in between.– For example, in a particular breed of chickens

called Andalusians, black and white offspring produce blue F1 hybrid offspring.

– Because neither the black nor white is dominant, capital or lower case letters are not used.

– Instead a C for color is paired with a superscript B for black or W for white.

Intermediate Inheritance:

CB CB CWCW CBCW

black

white blue

CB CB CWCW

CBCW

CWCWCBCWCBCWCB CB

X

The predicted phenotypes in F2 are 1 black: 2 blue: 1 white

Multiple Alleles:• Codominant: both alleles are expressed • The gene for human blood type expresses four phenotypes:

– A, B, AB or O.

• The alleles IA and IB are codominant and the i allele is recessive to both.

Phenotypes(Blood type)

Genotypes

A IAIA or IAi

B IBIB or IBi

AB IAIB

O ii

• The letters refer to two carbohydrates, designated A and B, which are found on the surface of red blood cells.

• Type O-negative blood does not have any antigens. It is called the "universal donor" type because it is compatible with any blood type.

• Type AB-positive blood is called the "universal recipient" type because a person who has it can receive blood of any type.

FYI:

Polygenic inheritance

• When a single phenotype results from the expression of two or more genes.

• For instance, phenotypes like high blood pressure are not the result of a single "blood pressure" gene with many alleles.

• The phenotype is an interaction between a person's weight, cholesterol level, kidney function, smoking, and probably lots of others too.

The Importance of Environment

• An individual’s phenotype depends on environment as well as genes.

• In human, nutrition influences height, exercise affects build, and exposure to sunlight darkens skin.

• The product of a genotype is generally not a single defined phenotype, but a range of possibilities influenced by the environment.

Chromosome Theory of Inheritance:

• It states that:– Genes are located on chromosomes.– Homologous Chromosomes separate during

meiosis so that alleles are segregated.– Chromosomes under go segregation and

independent assortment during meiosis.

Mendel’s Law of Genetics:

1. Alleles are different forms of the same gene that segregate during gamete formation (Law of Segregation)

2. Alleles of different genes segregate independently (Law of independent assortment)

Sex-linked Genes

• Located on a sex chromosome • In humans, most sex-linked genes are found on the X

chromosome, which is much larger than the Y chromosome.

• Discovered by Thomas Hunt Morgan.

Xr Y

XR XRXr

female

XRY male

XR XRXr

female

XRY male

P:

F1:

All the F1 offspring had red eyes.

Morgan mated a white-eyed male fly with a red-eyed female fly.

XXRXr

female

XRY

male

F1:

XR Y

XR XRXR

female

XRY male

Xr XRXr

female

XrY male

F2:

Then, he bred the F1 offspring together.

He got the classical 3:1 phenotypic ratio.However, none of the flies with white eyes were female.He concluded that the gene involved is only on the X chromosome.

Are you colorblind?????

Are you colorblind?????

Are you colorblind?????