Observing Patterns in Inherited Traits

Chapter 10

Before you go on…

Review the answers to the following questions to test your understanding of previous material.

1. Most organisms are diploid. What does this mean in terms of inheritance?

2. What is the smallest unit of inheritance, and how do they function?

3. What do you call alternate forms of a gene? How do these arise? Can there be more than two?

4. How are gametes produced, and how do you describe the genetic information that is normally contained in each?

5. Explain Independent Assortment, which describes the behavior of genes (on different chromosomes) during Meiosis.

6. Discuss linked genes, and the significance of crossing over. When and how does this happen?

Learning Objectives1. Explain the process Mendel used in his experiments with garden peas,

and what his results added to our understanding of inheritance.2. Construct a Punnett square demonstrating a monohybrid cross

between one homozygous dominant individual and one homozygous recessive individual. Then use this method to predict the genotypes and phenotypes of a cross between 2 of these offspring (F1 = first filial generation). Use correctly the terms: parental generation, first filial (F1) generation and second filial (F2) generation. What is the predicted phenotypic ratio in the F2 generation?

3. Construct a Punnett square demonstrating a dihybrid cross between heterozygotes for both traits. Predict the phenotypes that would result. What is the predicted phenotypic ratio? Is this a good illustration of the principle of independent assortment?

4. Discuss the effect on the phenotype for traits governed by codominant alleles, alleles which demonstrate incomplete dominance, and multiple allele systems. Provide some examples of each.

5. Differentiate between polygenic inheritance and pleiotrophy, with examples.

6. How is gender inherited, and what is the predicted phenotypic ratio? What are sex-linked genes: how are they inherited, how does one become a “carrier” for a sex-linked trait, and how do you predict the inheritance pattern for these genes?

Model Organisms

• Rapid Reproduction

• Produce large # of offspring)

• Few chromosomes

• Ease of propagation

Model Organisms

*

* Although not all of these code directly for the production of proteins.

Mendelian Genetics

• Described the variation in sexually reproducing species.

Gregor Mendel (1822-1884)

A botanist and mathematician

To this day, Mendel’s principles accurately describe the inheritance of traits.

p. 152

The Garden Pea Plant• Self-pollinating• True-breeding (normally self-

pollinating, so different alleles not introduced)

• Can be experimentally cross-pollinated

p. 153

Mendel’s Peas

What do these results tell us about allele dominance for these characteristics?

Mendel studied these 7 characters. How many traits were studied?

p. 154

Independent Assortment• Mendel recognized “units” of inheritance that

governed specific traits– e.g. pink or white flowers– We know these units as alleles

• Mendel concluded that the two “units” for the first character were to be assorted into gametes independently of the two “units” for the other character – This is true if the genes for the two characters are located on

different chromosomes – e.g. flower color and pod shape are not linked– In other words, homologous chromosomes separate and are

sorted into gametes at random during meiosis

p. 156

Genetic TermsA pair of homologous chromosomes(one from egg; one from sperm)

A gene locus (i.e. location)

A pair of alleles

Three pairs of genes

Dominant allele (Y)

will mask a recessive

allele (y) that it is

paired with as a

result of fertilization.

(Yy, YY, or yy)

Genotypes:

If both alleles the

same: homozygous

If alleles different:

heterozygous

What is the difference between the terms phenotype and genotype?

p. 153

Generation

Parental (P)

First Filial (F1)

Second Filial (F2)

True-breeding True-breeding yellow pea green pea (pollen) (eggs)

x

all yellow

grow plants,cross pollinate

allow to self-fertilize

6022 yellow : 2001 green

3 : 1

Monohybrid Cross: What did Mendel learn from this experiment?

p. 154

Alleles…

Pea color alleles

Y y

What color are the peas in this plant?

When this diploid plant produces a gamete, what allele(s) might the gamete contain?

If allowed to self-pollinate, and fertilization is random, what are the possible combinations of alleles in the resulting offspring, and what will their peas look like?

…on HomologousChromosomes

Genotype Phenotypep. 153

Punnett Square of a Monohybrid cross

Yellow x Yellow

YY x Yy = ?

Y Y

Y

y

YY YY

Yy Yy

Genotypic ratio?

Phenotypic ratio?

1:1, YY:Yy

4:0, yellow: green

During gamete formation, the alleles segregate from each other

p. 155

Punnett Square of a Monohybrid Cross

Female gametes

Male gametes

aA

aaAa

AaAAA

a

Aa X Aa

Genotypic ratio?

Phenotypic ratio?

1:2:1

3:1p. 155

Test Cross

Y? x yy

• the phenotype of the offspring will reveal the genotype of the unknown parent.

• If offspring result in ½ yellow and ½ green what is the parental genotype?

Pea with unknown genotype

y

y

Y Yy

Yy

yy

yy

y

p. 155

Complete Dominance

• Unattached earlobes

• Widow’s peak• Long eyelashes• Freckles• Cleft chin• Certain diseases:

e.g. cystic fibrosis

p. 153

Many traits are not expressed via complete dominance.

• Sex linked traits Which chromosome(s) carries these alleles?

• Incomplete dominance (p. 158)

Is this the same as “equally dominant” aka co-dominant?

• Polygenic inheritance (p. 158)

“poly” means….

• Pleiotrophy & Epistasis (p. 159)

Differentiate between these forms of gene expression.

• Environmental controls (p. 162)

Provide at least one example of how the environment can affect gene expression.

Sex linked genes

• Fathers pass sex linked alleles to all of their daughters but none of their sons. Mothers pass alleles to both.

X Y b X Xb b

–Color blindness–Hemophilia–Male pattern baldness p. 169, 175

Flower Color in Snapdragons: Incomplete Dominance

Pink-flowered plant X Pink-flowered plant

White-, pink-, and red-flowered plants in a 1:2:1 ratio

(heterozygote) (heterozygote)

p. 158

Incomplete Dominance

Hair texture in Caucasians

RR: curly

Rr: wavy

rr: straight

Curly hair is shaped like an elongated oval and grows at a sharp angle to the scalp.

ABO Blood Type:Co-dominance in a

Multiple Allele System

• Type A - IAIA or IAi

• Type B - IBIB or IBi

• Type AB - IAIB

• Type O - ii

Identify the blood type known as the:

Universal donor…

Universal recipient …

p. 158

Polygenic Inheritance & Continuous

Variation

The cumulative effect of multiple genes on one phenotype.

• Genes A, B, C each contribute a unit of darkness.

AABBCC: very dark

AaBbCc: intermediate darkness

aabbcc: very light

The cumulative effect of multiple genes on one

phenotype.

p. 160

Pleiotropy • Alleles at a single

locus may have effects on two or more traits

• Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia– Cell shape and

resistance to malaria

Why has this disease not been eliminated by natural selection?

p. 159

Epistasis: gene expression dependent upon two or more genes.

Melanin• BB = black lab• Bb = black lab• bb = brown lab

How much melanin?• EE = full deposition• Ee = full deposition• ee = no deposition

p. 160

Epistasis cont.• BBEE, BbEE, BBEe,

or BbEe = black

• BBee, Bbee, or bbee = yellow

• bbEE = chocolate• bbEe = chocolate

Must have at least one dominant allele at both loci for black pigment to be deposited

Pigment is not deposited

Brown pigment is deposited

Don’t for get the world around you

The expression of some genes is heavily influenced by environmental factors.

Acidity of the soil changes the color of hydrangea flowers.

p. 162

Don’t for get the world around you

• Being in the sun darkens our skin

• Ratio of red : white blood cells are affected by exercise, injury, and illness.

• Siamese cat and Himalayan rabbit: fur color is dependent upon temperature.