3.4 Distinguish among observed inheritance patterns caused by several types of genetic traits (dominant, recessive, incomplete dominance, codominant, sex-linked, polygenic, and multiple alleles).

3.5 Describe how Mendel’s laws of segregation and independent assortment can be observed through patterns of inheritance (such as dihybrid crosses).

3.6 Use a Punnett Square to determine the probabilities for genotype and phenotype combinations in monohybrid crosses.

Biology Content Standards 3. Genetics Broad Concept: Genes allow for the storage and transmission of genetic information. They are a set of instructions encoded in the nucleotide sequence of each organism. Genes code for the specific sequences of amino acids that comprise the proteins that are characteristic of that organism.

INTRODUCTION HEREDITY – the passing of traits from parent to offspring.

GENETICS – scientific study of heredity

A Blast from the Past! A GENE is a functional unit of heredity (piece of DNA ) carried on a CHROMOSOME.

HEREDITY -

GENETICS -

GENE -

DNA -

CHROMOSOME -

Let’s Review the passing of traits from parent to offspring.

scientific study of heredity.

a functional unit of heredity (piece of DNA ) carried on a CHROMOSOME.

a code used within cells to form proteins. A threadlike linear strand of DNA and associated proteins in the nucleus of eukaryotic cells that carries the genes and functions in the transmission of hereditary information.

A circular strand of DNA in bacteria that contains the hereditary information necessary for cell life.

GREGOR MENDEL ( priest, scholar, teacher, scientist ) 1822 - 1884

He spent much of his time in the monastery garden working primarily with the garden pea. Through his experiments, Mendel discovered the basic principles of heredity!

Pea plants reproduce by: SELF-POLLINATION The pollen (sperm cells) produced by the plant fertilizes the egg cells of the very same plant.

Mendel had received a stock of PUREBRED peas developed by earlier gardeners. These peas produced offspring that were identical to themselves.

Mendel and his Pea Plants

Pea plants can also reproduce by CROSS-POLLINATION. The pollen (sperm cells) produced by one plant fertilize the egg cells of another plant.

Mendel and his Pea Plants

Mendel had several

types of pea plants in

his garden and kept

careful records of the

inheritance of 7 clearly

contrasting pairs of

TRAITS (characteristics).

Mendel and his Pea Plants

Mendel and his Pea Plants

Refer to p.264

Mendel crossed pea plants with different characters for the same trait. He produced HYBRIDS – organisms produced by crossing parents with differing characters. Mendel found that all of the offspring had the character of only ONE of the parents!

Mendel and his Pea Plants

GENES and DOMINANCE

1.  Each of the traits Mendel studied was controlled by one gene that came in two different forms. (ex) - Height gene comes in Tall and Short. 2. ALLELES – the different forms of a gene. 3. Some alleles are DOMINANT: mask the other allele (ex) Tall

Others are RECESSIVE: hidden by the dominant allele (ex) Short

Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that

are hybrid for a trait will display only the dominant trait.

Mendel now asked, “What happened to the recessive characters?”

and allowed all 7 kinds of hybrid plants to self-pollinate…

Hybrid

1st Filial Generation

F1

F2

3 Smooth : 1 Wrinkled

2nd Filial Generation

Refer to p.265

GG gg

Gg

Gg GG gg

The F1 Cross à The recessive characters had not disappeared!

The recessive characters reappeared in the F2 generation.

Why?

LAW OF SEGREGATION A pair of alleles is segregated, or separated, during the formation of gametes.

B

b

PUNNETT SQUARE – a diagram used by biologists to represent genetic crosses. (EX) Tt x Tt

Gametes Potential offspring

Monohybrid Cross A type of Single Trait Cross

Genotype Ratio 1 SS: 2 Ss: 1 ss Phenotype Ratio 3 Smooth: 1 Wrinkled

Homozygous SS, ss

Heterozygous Ss

S = smooth

s = wrinkled

Ss x Ss

Click here for a great site with Punnett square problems!

Mrs. Bailey’s 4 Key Steps to Solving a Punnett Square!

1. Write the key (ex) G = green g = yellow

2. Write the cross (ex) Gg x Gg

3. Punnett Square

4. Answer the question!

Mendel now asked the question, “Does the segregation of one pair of alleles affect the segregation of another pair of alleles?” For example, Must a round seed be yellow?

Law of Independent Assortment

Factors for different characteristics are distributed to gametes independently.

Key: R = Round r = wrinkled Y = Yellow y = green Cross:

RrYy x RrYy

Dihybrid Cross A type of Double Trait Cross Refer to p.271

Now let’s solve some Double Trait Crosses!

Click here for a great site with Punnett square problems!

Other Types of Crosses •  Test cross

•  Incomplete Dominance

•  Codominance

Testcross An individual of unknown genotype is crossed with a homozygous recessive individual.

Incomplete Dominance Occurs when 2 or

more alleles influence

the phenotype.

Results in a phenotype

intermediate between

the dominant trait and

the recessive trait.

Example: Pink four o’clocks are allowed to self-pollinate.

RR = red Rr = pink rr = white

Rr X Rr

Genotype Ratio:

Phenotype Ratio:

Let’s solve …

Both alleles for a gene are expressed in a heterozygous offspring. Codominance

For example, red cows crossed with white will generate roan cows.

Roan refers to cows that have red coats with white blotches.

Example: Roan coat color in a horse – The genes for both red coat color and white coat color are expressed

RR = Red coat color Rʹ′R' = White coat color RRʹ′ = Roan coat color

RRʹ′ X RRʹ′

Genotype Ratio:

Phenotype Ratio:

Let’s solve …

Example: Roan coat color in a horse – The genes for both red coat color and white coat color are expressed.

HR HR = Red coat color HWHW = White coat color HRHW = Roan coat color

HRHW X HRHW

Genotype Ratio:

Phenotype Ratio:

Let’s solve …