Introduction to Genetics

10.2 and Chapter 11Biology I

2009

Comprehension CheckBrain Teaser:

How can two brown rabbits produce all white offspring?

What color of offspring would two white rabbits produce

History of Genetics

• Genetics: the science of heredity

• Gregor Mendel: the first scientist to study patterns of genetics– Mendel was a monk

who studied pea plants– He cross pollinated

plants to see the outcome of mixing different traits

Genetics Vocabulary

• Genotype versus Phenotype– A genotype is the genes that an organism has

embedded in their DNA– A phenotype is the physical trait seen when

looking at an organism

• F1 Generation versus F2 Generation– F1 organisms are offspring of two parents– F2 organisms are offspring of two F1

organisms

Genetics Vocab Continued• What is the difference

between a “chromosome” and a “gene”?– A chromosome is tightly

coiled DNA found in the nucleus of eukaryotic organisms

– Humans have 46 chromosomes in each body cell

– A gene is a region of DNA found on a chromosome that controls a trait, for example you have a gene for eye color

• What is an allele?– An allele is different forms of

a trait, for example brown eyes or blue eyes

Genetics Vocab Continued

Dominant Alleles versus Recessive Alleles• Homozygous Dominant Organisms

• Homozygous Recessive Organisms

• Heterozygous (Hybrids) Organisms

?

Genetics Problems Rules

1. Always “assign” letters to represent the alleles

2. Read the problem thoroughly to determine the parents’ genotypes.

3. Solve the problem (predict the genotypes of the offspring) using a Punnett Square.

Complete Dominance Inheritance

Guidelines:– One allele is dominant over the other – The dominant allele must be written as

a capital letter– If heterozygous, the capital letter is

always written first. – Choose letters where the capital letter

and lower case look different from each other. For example Rr, Tt, Bb, NOT Oo Jj

Complete DominanceProblem #1:In birds, a straight beak is dominant over a curved beak. A

heterozygous male is crossed with a homozygous recessive female. What are

the predicted genotypic and phenotypic ratios of their offspring?

Complete DominanceProblem #2: Cross 2 heterozygous parents. What are the predicted genotypic

and phenotypic ratios of their offspring?

Incomplete Dominance Inheritance

Guidelines• Certain traits have no gene expression that will be dominant.

In these cases, the traits expressed will blend together. • Example: In corn, kernel color shows incomplete dominance.

Yellow and brown blend to form orange kernels. – Use 2 CAPITAL letters to represent the alleles:

Y = Yellow alleleB = Brown allele

Genotype PhenotypeYYBBBY

Incomplete Dominance

Problem #1: Cross an orange kernel corn plant with a corn plant that has yellow kernels. What are the predicted genotypic and phenotypic ratios of their offspring?

Self-Assessment

• In teachers from Illinois, the trait for being boring is dominant over the non-boring trait. Cross Mr. Smithwell, who is homozygous dominant, with a heterozygous teacher. What are the genotypes and phenotypes of their children?

• In tulips, red flowers mix with white to form pink. A cross between two flowers produces all red offspring. What were the genotypes of the parent flowers?

Multiple Allele Inheritance Guidelines:• With multiple alleles more than two genes control a particular

trait. • The most common example of this is blood type.• There are 3 alleles and 4 blood types (phenotypes).• The alleles are:

A B O

• A and B are both dominant over O, but A and B are not dominant over each other.

GENOTYPE PHENOTYPEAAAOABBBBOOO

Multiple AllelesProblem #1: Mr. Wiggles is trying to prove that he is not the father of 500

bunnies. He has a blood type of O. 50% of the bunnies have blood type A and 50% of the bunnies have blood type AB. Can Mr. Wiggles be the father of these bunnies?

1. If heterozygous, the capital letter is always written first.

Pedigrees• Pedigrees can be used to track

genetic information through generations of a family

• Pedigrees have a few simple rules:– Men are shown as squares– Woman are shown as circles– Each generation is given a Roman

Numeral (I, II, III, IV, V)– Each individual within a generation is

given a number (1, 2, 3, 4, 5)– Shaded individuals have the trait being

tracked in the pedigree

PedigreesProblem #1: The trait shown below is the ability to taste PTC

(phenylthiocarbamide) paper. This trait is controlled by a dominant gene represented by T, and is transmitted by normal inheritance. Nontasters are, therefore, Homozygous for the recessive trait and are represented by tt. The shaded figures below are both homozygous recessives (tt) and are nontasters. All unshaded symbols have two possible genotypes: TT or Tt. Determine the genotypes of all the individuals in the pedigree below:

Pedigrees

Problem #2• Write the correct Roman

numeral for each generation.• Write the correct number for

each individual.• Assume the shaded symbols

represent the recessive homozygous genotype rr. Which, individuals show the homozygous recessive trait?

• In the spaces below each symbol, write as much of the genotype of each individual as can be determined from the information provided.

Genetics AColorblindness in humans is caused by a sex-

linked recessive gene on the X-chromosome. In this pedigree chart, both of the first generation parents are colorblind. Assuming that none of the in-laws are colorblind or are carriers, what is the maximum number of descendents that could be colorblind?

a. 3b. 4c. 5d. 12

Genetics AColorblindness in humans is caused by a sex-

linked recessive gene on the X-chromosome. In this pedigree chart, both of the first generation parents are colorblind. Assuming that none of the in-laws are colorblind or are carriers, what is the maximum number of descendents that could be colorblind?

a. 3b. 4c. 5d. 12

Sex-Linked Traits• Sex-linked genes are found on the X chromosome but NOT

on the Y chromosome. WHY?• Sex-linked traits are usually recessive. • Males are more likely to express a sex-linked trait. WHY?• Example: Baldness• GENOTYPE PHENOTYPE

XBXBXBXbXbXbXBYXbY

Sex-Linked Traits• Cross a carrier female with a bald man. Predict how many of their children

will be bald:

Sex-Linked Traits• Cross a bald female with a normal male. Predict how many of their

DAUGHTERS will be bald.

Karyotypes• Karyotypes are a

way of looking at chromosomes for genetic disorders.

• You can not see changes in genes, but you can see changes in chromosome number.

• Is this karyotype from a male or female?

Genetics BRed-green colorblindness is an example of a

genetic disease caused by a sex-linked recessive gene located on the X-chromosome. In this pedigree chart, the first-generation father is colorblind and is not a carrier. How many of the original mother and father’s children and grandchildren could be colorblind?

a. 2b. 4c. 6d. 8

Genetics BRed-green colorblindness is an example of a

genetic disease caused by a sex-linked recessive gene located on the X-chromosome. In this pedigree chart, the first-generation father is colorblind and is not a carrier. How many of the original mother and father’s children and grandchildren could be colorblind?

a. 2b. 4c. 6d. 8