Heredity Part 1: Genetics

Biology 10.1

Gregor Mendel

• 1800’s Austrian (NOT AUSTRALIAN) monk, mathematician, and scientist• Began working in the garden at his monastery

• Father of Genetics• His work led to an understanding of how traits are passed from parents to

offspring• The passing of traits from parents to offspring is called heredity

• Primarily worked with a variety of garden peas• Noticed several traits that appeared to follow certain patterns of inheritance.

Mendel’s Pea Plant Traits

Mendel’s Observations• Mendel mated pea plants together until he got plants that were true-

breeding• Also known as pure-bred

• Organisms that always pass on a certain trait• Ex. Purple plant that always has purple offspring

• Once he had established 2 true-breeding lines for each trait, he mated them together• For example, true-breeding yellow seed and true-breeding green seeds.• He noticed that all of the offspring looked like one of the parents, but not the

other (Ex. All yellow, no green)• Called them hybrids because they got one gene from each parent

Generations in Crosses

• P=Parental generation• First ones to be crossed

• F1=1st Filial generation • Filial=children

• F2=2nd Filial generation• Grandchildren

Mendel’s Observations continued

• He then self-pollinated each of the offspring• He noticed that most looked like the parent, but some did not

Mendel’s Hypotheses

• He hypothesized that one of the traits was overpowering the other• He called these traits dominant

• Because the other trait was not expressed in the first generation, but reappeared in the second, he hypothesized that the trait was still there, but was being overpowered by the dominant trait• He called these traits recessive

Example of Mendel’s Experiment-Yellow and Green seeds

• Mendel developed a true-breeding line of plants with yellow seeds and a true-breeding line of plants with green seeds.• He took one yellow-seeded plant and one green-seeded plant

and mated them together.• All of the offspring had yellow seeds

• He then took each of these yellow-seeded plants, and self-pollinated them• ¾ of the offspring had yellow seeds and ¼ of them had green seeds.

• He concluded that Yellow was dominant and green was recessive

Results of Mendel’s Experiment

• Mendel’s experiments continually showed that ¾ of the plants showed the dominant trait, and ¼ showed the recessive trait• 3:1 ratio

Mendel’s Laws of Inheritance

• Law of Segregation• For any particular trait, the pair of alleles from each parent separate and only

one allele passes from each parent to an offspring• Recall that we have 23 pairs of chromosomes (one set from mom and one set from dad)

• During meiosis, one of these are sorted into each gamete (sex cell)• Which one winds up in each is random.

• Law of Independent Assortment • Different pairs of alleles for one trait are passed to offspring independently of

alleles for other traits• Most traits are not linked in any way. You could get your mom’s blue eyes without getting

her blonde hair, height, etc.

Mendel’s Principle of Dominance

•Alleles can be dominant or recessive• Dominant alleles mask, or cover up, recessive alleles• Recessive alleles can only be expressed if the offspring

receives 2 of the recessive alleles• Any time a dominant allele is present, it will be expressed

Symbols for alleles

•Dominant alleles are represented by capital letters• Example: A, B, C, D

•Recessive alleles are represented by lowercase letters• Example: a, b, c, d

• The gene is represented by a single letter, and dominant and recessive alleles of the gene are shown by whether they are capital or lowercase

You try it: Indicate whether the following are dominant or recessive1. T2. d3. P4. E5. e

You try it: Indicate whether the following are dominant or recessive1. T-dominant2. d-recessive3. P-dominant4. E-dominant5. e-recessive

Homozygous vs Heterozygous

• Recall that organisms get 2 alleles-1 from each parent• Homozygous-2 of the same alleles for a trait• “Homo” means same• Example: TT or tt• Can be homozygous dominant (TT) or homozygous recessive (tt)

• Heterozygous- 2 different alleles for a trait• “Hetero” means different• Example: Tt or tT

• Note: The latter is rarely used, but will sometimes be the result in a Punnett Square

You Try It: Indicate whether the following are heterozygous, homozygous dominant, or homozygous recessive.

• TT• Tt• tT• tt

You Try It: Indicate whether the following are heterozygous, homozygous dominant, or homozygous recessive.

• TT-homozygous dominant• Tt-heterozygous• tT-heterozygous• Tt-homozygous recessive

Punnett Squares

• Tool to predict results of a cross (mating) • Usually a 2x2 grid• Can be larger if including more traits

• Couple different types• Monohybrid cross (one trait)• Dihybrid cross (2 traits)• Trihybrid cross (3 traits)-Gets too time consuming for this class• And more that are too complicated to be done by hand and are instead done

using computers

Punnett Squares Show Probability

• Not actuality • Coin flip• If you flip a coin, what is the chance that it comes up heads?

• 50%• If you flip it again, what is the chance it will come up heads a second time?

• 50%• And so on

• If there is a 1 in 4 chance that your offspring will have blue eyes, and you have 3 children with brown eyes, will the 4th for certain have blue eyes?

Using Punnett Squares Draw example• Begin by drawing a 2x2 grid• Place one parent’s alleles on top• Place the other parent’s alleles

on the left side• Bring ones on the top down • Bring one on the left across

aA A

a

A

A

A

A

a

a a

a

Practice Problem

• A pea plant with genotype Rr is mated with a pea plant with genotype rr. What genotypes could the offspring have?

Practice Problem

• Begin by drawing a 2x2 grid• Place one parent’s alleles on top• Place the other parent’s alleles

on the left side• Bring ones on the top down • Bring one on the left across

rR r

r

R

R

r

r

r

r r

r

Genotype and Phenotype

• Genotype-The genes (alleles) that an organism has• Geno=Gene• Example: Aa, BB, cc

• Phenotype-The expression (appearance) of the genotype• Ph-Physical• Example: Green eyes, brown hair, tall

You try it: Indicate whether the following are genotypes or phenotypes.• Aa• Blue eyes• Rolling tongue• TT

You try it: Indicate whether the following are genotypes or phenotypes.• Aa-genotype• Blue eyes-phenotype• Rolling tongue-phenotype• TT-genotype

Principles of heredity

1. Traits are controlled by alleles (different versions of a gene) on a chromosome

2. An allele can be dominant or recessive3. When a pair of chromosomes separate during meiosis, the different

alleles for a trait move into different gametes

Practice Problem #1

• A person with brown eyes (BB) has a child with a person with blue eyes (bb). What is the chance that the child will have blue eyes?

Practice problem #2

• 2 people with brown hair (BB) have a child. What is the chance that they will have a child with brown hair?

Practice Problem #3

• A heterozygous brown dog mates with a homozygous recessive white dog. What is the chance that their puppies will be white?

Practice Problem #4

• Tongue-rolling is a dominant trait. If two people heterozygous for tongue-rolling have a child, what is the chance that it will not be able to roll its tongue.

Practice Problem #5

• Red hair is recessive to all other hair colors. Can 2 people with brown hair have a child with red hair?

Dihybrid Crosses

• Deals with 2 traits• Looking at 2 genes, so 4 possible alleles

• Little bit more complicated• Punnett Square will be 4x4

• 16 possible offspring combinations

• Principle of Dominance is still followed• F1 generation will still show dominant phenotype• F2 generation will have a ratio of 9:3:3:1

• http://www.showme.com/sh/?h=wBNbIVE

How to set up Punnett Square

• One allele from each trait will join with one for the other trait, and will give 4 allele combinations

• EX: AaBb AB, Ab, aB, ab

• Do for both parents• Draw a 4x4 grid• Place alleles for one parent on top, the other on the side• Solve like other Punnett Square, only make sure to bring both alleles

down and across

Example of Dihybrid Cross

• If you cross a pea plant with genotype Rryy and one with rrYY, what are the possible genotypes of the offspring?• 1st plant allele combinations: Ry, Ry, ry, ry• 2nd plant allele combinations: rY, rY, rY, rY• Draw 4x4 grid• One parent’s alleles on top, the other on the left side

You try it!

• Tall is dominant to short and Wide is dominant to narrow• If you crossed 2 heterozygotes, what number of the offspring would

display each of the phenotypes?• Tall wide: _________________• Tall narrow: _______________• Short wide: _______________• Short narrow: _____________