6.2 Principles of Inheritance

You’ll be able to:

1. To explain the chromosome theory of heredity

2. To summarize Mendel’s 3 laws

Walter S. Sutton

• Born 1877, Utica, N.Y., • U.S. geneticist • 1902 Demonstrated

Gregor Mendel’s concept of heredity.

• In 1903 concluded that chromosomes contain units of heredity

Chromosomal Theory of Heredity

• Inheritance of traits is controlled by genes

• Genes are located on chromosome

Each color represents a different gene

Representing Alleles

• Designated by capital and lower cases pairs

• Capital letters = dominant allele

• Lowercase letters = recessive alleles

• A = dominant

• a = recessive

Genotype vs. Phenotype

Genotype• Describes the genes

inherited by an organism.

• Represented by two letters

Examples• AA, Aa, aa

Phenotype• An individual's

anatomical structure, physiology and behavior.

• Refers to everything that can be easily observed and measured about an plant, animal or human being.

Homozygous vs. heterozygous

Homozygous• Same alleles• Purebred

Genotypes: AA or aa

Heterozygous• Different alleles• Hybrid

Genotypes: Aa

Genotype Symbol Genotype Vocab Phenotype

TThomozygous DOMINANT

or pure tall

tall

Ttheterozygous

or hybrid

tall

tthomozygous RECESSIVE 

or pure short

short

Mendel’s 3 Laws

1. the Law of Dominance

2. the Law of Segregation

3. the Law of Independent Assortment

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 have only the dominant trait in the phenotype.

   

Parent Pea Plants(Purebred parents with

opposite traits)

F1 Pea Plants(their offspring)

tall stem x short stem all tall stems

yellow seeds x green seeds

all yellow seeds

green pea pods x yellow pea pods

all green pea pods

round seeds x wrinkled seeds

all round seeds

axial flowers x terminal flowers

all axial flowers

•All offspring express the dominant allele in their phenotype!!!!!

Punnett Square Proof

• Parents (P):  TT x tt • T = the dominant allele for tall stems • t = recessive allele for short stems

Parent Pea Plants  (P Generation)

Offspring  (F1 Generation)

Genotypes: TT x tt

Phenotypes: tall x short

Genotypes: 100% Tt

Phenotypes: 100% tall

Law of Segregation

• During the formation of gametes (eggs or sperm),

• the two alleles responsible for a trait separate from each other. 

• Alleles for a trait are then "recombined" at fertilization,

• producing the genotype for the traits of the offspring.

Example

•When you "split" the genotype letters & put one above each column & one in front of each row, you have SEGREGATED the alleles for a specific trait.

•In real life this happens during a process of cell division called "MEIOSIS". 

•Meiosis leads to the production of gametes (sex cells), which are either eggs or sperm.. 

•If you cross two hybrid:

• 3/4 boxes will produce an organism with the dominant trait ("TT", "Tt", & "Tt")

•1/4 boxes ends up homozygous recessive ("tt").

Parent Pea Plants  (Two Members of F1 Generation)

Offspring  (F2 Generation)

Genotypes: Tt x Tt

Phenotypes: tall x tall

Genotypes: 25% TT 50% Tt 25% tt

Phenotypes: 75% tall

25% short

Law of Independent Assortment

• Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

The genotypes of our parent pea plants will be:

RrGg x RrGg

• "R" = dominant allele for round seeds

• "r" = recessive allele for wrinkled seeds

• "G" = dominant allele for green pods

• “g" = recessive allele for yellow pods

RrGg x RrGg

• Notice that we are dealing with two different traits:

• (1) seed texture (round or wrinkled)

• (2) pod color (green or yellow). 

• Notice also that each parent is hybrid for each trait (one dominant & one recessive allele for each trait).

• We need to "split" the genotype letters & come up with the possible gametes for each parent. 

• Keep in mind that a gamete (sex cell) should get half as many total letters (alleles) as the parent and only one of each letter.

• So each gamete should have one "are" and one "gee" for a total of two letters. 

• There are four possible letter combinations: • RG, Rg, rG, and rg. • These gametes are going "outside" the p-

square, above 4 columns & in front of 4 rows. 

• We fill things in just like before --- "letters from the left, letters from the top". When we finish each box gets four letters total (two "are's" & two "gees").

  RG Rg rG rg

RGRRGG round

RRGg round

RrGG round

RrGg round

RgRRGg round

RRgg round

RrGg round

Rrgg round

rGRrGG round

RrGg round

rrGG wrinkled

rrGr wrinkled

rgRrGg round

Rrgg round

rrGg wrinkled

rrgg wrinkled

The results from a dihybrid cross are always the same:

•9/16 boxes show dominant phenotype for both traits (round & green)

•3/16 show dominant phenotype for first trait & recessive for second (round & yellow)

•3/16 show recessive phenotype for first trait & dominant form for second (wrinkled & green)

•1/16 show recessive form of both traits (wrinkled & yellow).

Results

• a green pod can have round or wrinkled seeds

• a yellow pod can have round or wrinkled seeds

• The different traits do not influence the inheritance of each other. 

• They are inherited INDEPENDENTLY.

Summary

LAW PARENT CROSS OFFSPRING

DOMINANCETT x tt tall x short

100% Tt tall

SEGREGATIONTt x Tt tall x tall

75% tall 25% short

INDEPENDENT ASSORTMENT

RrGg x RrGg round & green x round & green

9/16 round seeds & green pods

3/16 round seeds & yellow pods

3/16 wrinkled seeds & green pods

1/16 wrinkled seeds & yellow pods