Mendel’s Laws of Heredity

Mendel’s Laws of Heredity

Section 10.1 p. 253 - 262Chapter 10 Mendel and

Meiosis

• Mendel chose which pea plants he would allow to fertilize or pollinate one another and then kept records of the offspring produced.

Before we can go any further…

• There are a bunch of vocab words we need to start using.– Alleles– Dominant– Recessive– Homozygous– Heterozygous

Mendel’s ~ The Rule of Unit Factors

• Each organism has two factors that control each of its traits.

• These factors are now known as genes.

• Genes come in different forms.

• Alternative forms of a gene for each variation of a trait is an allele.

alleles

• We get two copies of each allele or directions for a trait.

• One copy comes from each parent.

• We may get a matching set or they may be different with the dominant one hiding or masking the recessive version.

dominant

• Observed trait of an organism that masks the recessive form of a trait.

• If present, this is the trait we see.

• “Dominant” is not always “good”.

recessive

• Trait of an organism that can be masked by the dominant form of a trait.

• You need two copies, one from each parent, for a recessive trait to be expressed.

• Recessive traits are not always “bad”.

Mendel’s ~ The Rule of Dominance

• Even though an organism carries two alleles for the same trait, only one trait is observed.

• The dominant trait will be expressed.

Homozygous

• Having two of the same alleles for a gene

• When there are two identical alleles for a trait.

Heterozygous

• Having two different alleles for the same gene.

• Back to the peas, each pea has two height genes.

• T = tall, t = short

• TT and tt are homozygous.

• Tt is heterozygous.

Phenotype

• Outward appearance of an organism, regardless of its genes.

• An organism's traits (like green vs. yellow seeds, or pea plant height)

• What you look like!

Genotype

• Combination of genes in an organism.

• Can not tell from outward appearance

• Set of directions on the inside.

Genotype

• If it is a recessive trait, the genotype is obvious.

• Dominant traits are expressed whether the organism is heterozygous (2 different alleles) or homozygous (2 of same alleles) so you do not know for sure just from looking.

hybrid

• Offspring formed by parents having different forms of a trait.

• Mendel’s crossbreed strains were made breeding know true-breeding plants to different true-breeding plants.

• Working with his peas, Mendel used 355 cross-bred strains and 12,980 resultant hybrids.

True-breeding or homozygous

• True-breeding means that they only have one version of a trait to pass on or homozygous.

• Sometimes we refer to homozygous organisms as being a “pure bred”.

• If a pea plant came from parents that were tall, it is tall and all of its offspring are tall when crossed with other pea plants that are “true-breeding”, we can be fairly certain that it is homozygous.

Poodles

Wolves

• What would their puppies look like?

• In 1976 a person named C. Schleifenbaum lead a study to find out.

poodle-wolf hybrids

• They bred wolves to poodles.

• Then looked at the fur marks and coat color in wolves and poodle-wolf hybrids.

• Here are some pictures of their results.

1st generation ~ poodle-wolf hybrid

Notice how the coats range from wolf-like to poodle-like

2nd generation ~ poodle-wolf hybrid

Monohybrid cross

• How can we explain the poodle-wolf hybrid results?

• Poodles are “true-breeding” for one version of traits.

• Wolves have a different version of these same traits.

Monohybrid cross

• In a monohybrid cross, the parents selected are true-breeding for the trait we will be studying.

• The parents are known as the P1

generation.

• Two P1 parents will be breed or crossed and the resulting offspring are the 1st or F1 generation.

1st generation

• First generation is the F1 generation

• “F” stands for filial ~ son or daughter

• In the F1 generation, all of the offspring should be similar, showing all the dominant traits that were passed on by the P1 generation.

1st generation

• All of the poodle-wolf hybrids looked the same.

Mendel’s ~ The Law of Segregation

• Each plant has two different alleles, it can produce two different types of gametes.

• During fertilization, male and female gametes randomly pair to produce four combinations of alleles.

The Law of Segregation

Homozygous P1 =

All offspring will have the same genotype

They will all have the same phenotypes & look like the dominant trait.

Heterozygous P1 =

The offspring will have a 1:2:1 ration in genotypes

3:1 ration in phenotypes

Monohybrid Crosses

Homozygous crosses

• Mendel choose plants that were breeding true.

• His peas must have been homozygous…

• Only able to provide one type of allele or variety of that particular trait.

Homozygous crosses

• Mendel’s first pea plant crosses were between tall pea plants and short pea plants.

• One parent has two tall alleles and the other parent has two short alleles.

• They are both homozygous for their trait.

How we represent the parents & offspring in a

cross• A letter is selected to represent a

trait.

• The dominant version is shown with the capital.

• The recessive version is shown with the lower case .

How we represent the parents in a cross

• With the height of pea plants a capital T is used to represent the tall version

• A lower case t is used to represent the short version

Genotypes in the cross

• The tall pea parents will be = – TT, it can only give T alleles

• The short pea parents will be =– tt, it can only give t alleles

Punnett Squares

• A punnett square is a way to predict the possible outcome of a breeding cross

• Each box represents a reproductive opportunity or offspring.

Short parent’s alleles

t

tTall parent’s alleles

T

T

Punnett Squares

• The offspring gets one allele from each parent so they end up with two copies of alleles for each trait.

• Divide each parent’s two alleles so that each box has one letter next to or above it


t


T

T

Monohybrid cross - Homozygous

• If one parent is homozygous dominant for their trait and the other is homozygous recessive, the offspring will all have the same phenotype and genotype.


t


T

T

• The offspring’s genotype (what the alleles say) will be =

• Tt• The offspring’s

phenotype (what we see) will be =

• The plants will be tall.

• The ________ is the dominant trait.


t


T

T

Monohybrid cross - Homozygous

T

Monohybrid cross

Both Homozygous/

1 –Recessive

1 -Dominant

t

T t

t t

t

t

T

T

T

TGenotype

Genotype

Genotype

Genotype

Phenotype =

Phenotype =

Phenotype =

Phenotype =

Dominant

DominantDominant

Dominant

2nd generation

• Second generation is the F2 generation

• The second generation all has two

different copies (heterozygous) for each trait.

• Their offspring will show more variation because the alleles will regroup in many ways.

Monohybrid cross - Heterozygous

• If two organisms are selected out of the F1 generation to be parents for the next generation, they will each be heterozygous.

• Sticking with our offspring of a tall pea plant & and a short pea plant, this group will have a genotype of Tt.

• They will have a phenotype of tall.


• Remember, the offspring get one allele from each parent so they end up with two copies of alleles for each trait.

• Both parents are heterozygous so they have a genotype of Tt.

Heterozygou

s parent’s alleles

T

tHeterozygou


T

t


• How many of the offspring will be tall?

• How many of the offspring will be short?

Heterozygou


T

tHeterozygou


T

t


• What are the possible genotypes for the offspring?

• What is the ratio of tall (dominant) to short (recessive) plants in the F2 generation?

Heterozygou


T

tHeterozygou


T

t

T

Monohybrid cross

Both Heterozygous

T

t T

T t

t

t

t

T

t

TGenotype

Genotype

Genotype

Genotype

Phenotype =

Phenotype =

Phenotype =

Phenotype =

Dominant

Recessive

Dominant

Dominant

2nd generation

• The second generation of poodle-wolf hybrids looks very different from each other!

Crossing Homozygous Pea Plants

Crossing Heterozygous Pea Plants

Documents

Mendel’s Laws of Heredity