Mendelian Genetics

GeneticsStudy of heredity, or the

passing on of traits (characteristics) from parent to offspring.

Important concepts for review

• Meiosis divides the genes found on homologous chromosomes from parent cells into separate gametes.

Important concepts for review

• Fertilization combines genes found in opposite sex cells in order to form new offspring with half of each parent’s traits.

How was genetics discovered?

• Genetics was discovered by Gregor Mendel. So he is known as the Father of Genetics.

Some Terms

• True-breeds – organisms that produce offspring identical to themselves if they self pollinate

• Hybrids – offspring of crosses between parents with different traits

• P generation – the parents of a cross

• F1 generation – the offspring of the P generation

• F2 generation –the offspring of the F1 generation

How was genetics discovered?

• Gregor Mendel observed traits or characteristics of the garden pea.

• Some are short-stemmed, some seeds are round, some are yellow...

• He observed that different pea traits are produced by different varieties of parent plants.

How do traits get inherited?

Pea plants self-pollinateResulting in true-breeds – pea plants

with only one form of a characteristic: only the short allele, or only round seeds, only the yellow pea pod color…

Mendel controlled the pollination of two true breed pea plants.

Mendel’s ExperimentsWhat would result if a true breed yellow pea plant was

crossed with a true breed green pea plant?

What happened to the yellow?

Did yellow disappear or is green simply dominant?

Mendel then crossed the F1 generation that resulted from the previous cross…

Yellow did not disappear; green is dominant

Warm-up:

• 1. What is phenotype?

• 2. What is genotype?

Lesson EQ

• What does Mendel’s Law of Segregation state?

*What did he realize?*

Key terms highlighted1. There is some hidden factor that controls

inheritance. It’s called a gene.

2. Genes can be of alternate forms called alleles. E.g. one form of the gene that determines pod color was green; the other was yellow.

3. Alleles may be dominant or recessive. The dominant trait is expressed or shown while the recessive is hidden.

*What did he realize?*Key terms highlighted

Alleles are carried on opposite homologous chromosomes.

We’ll label the dominant allele with a capital letter P and the recessive allele with a lower case p.

4. Organisms inherit two alleles for each trait. One from each parent. Allele combinations are either homozygous (both alleles are identical; AA or aa) or heterozygous (alleles are different; Aa).

5. Gametes play a role in genetics. Sex cells only carry one allele each because alleles segregate into separate gametes during meiosis. This is known as the Law of Segregation.Watch the segregation of A, a, B, b genes by clicking here

*What did he realize?*Key terms highlighted

Through Mendel's observations we are now able to predict the probability of the genes a offspring may have. Base upon

the parents traits.

*Predicting genetic outcomes*

Monohybrid crossA Punnett Square is used to predict the probability of genetic outcomes. Here’s how…

•The square has 2 columns and 2 rows

•Each row and column represents one of the 2 possible alleles carried by the sex cells of each parent (i.e. accounting for the 50% probability of inheriting from either gamete).

*Predicting genetic outcomes*

1. The dad’s genotype is segregated on top of the square

2. The mom’s genotype is segregated on the side of the square

*Predicting genetic outcomes*

3. The 4 boxes in the square are filled in with the gene type from dad’s gametes (B)

4. The 4 boxes in the square are filled in with the gene type from mom’s gametes (b)

*Predicting genetic outcomes*

Probability of inheriting traits:

•Of the four gametes produced by meiosis, two contain one of the homologous chromosome pair and two contain the other chromosome of the homologous pair

•Thus, if the parent was heterozygous for gene “A” (Aa) there’s a 50% chance that A will end up in the gamete and a 50% chance that a will end up in the gamete. Likewise for the B gene.

Warm-up• Cross a heterozygous tall plant with a short

plant. What is the probability that one of the offspring will be short?

Today

• Mendelian Genetics – Probability

• Benchmark Review! Cells and Kingdoms Due Today!

• Friday’s warm-up needs to be corrected. I asked you what The Law of Independent Assortment was when I should have asked you what the Law of Segregation was.

*Predicting genetic outcomes*

Accounting for probability in the monohybrid cross (Punnett square):

•50% chance of inheriting 1 of 2 gamete types from father (columns)

•50% chance of inheriting 1 of 2 gamete types from mother (rows)

•Thus, there’s a 25% chance that sperm 1 fertilizes egg 1

True-breeding yellow pod plants have 2 recessive alleles (gg) for pod color.

True-breeding green pod plants have 2 dominant alleles (GG) for pod color.

The resulting plants are hybrids (Gg).

What did Mendel’s cross look like?

*Allele Expression*Genotype

(allele type or “ingredients”)

Phenotype(expression or “cake”)

TT(homozygous=same)

Tall(Dominant)

Tt(heterozygous=diff.)

Tall(Dominant)

tt(homozygous)

Short(Recessive)

Try this one…

The ability to roll the tongue is dominant over the inability to do so in humans.

(A) If two heterozygous tongue- rollers have children, what genotypes could their children have?

Hint: T=tongue-rolling and t=non-tongue-rolling

A little different….• A man and a woman are heterozygous for

freckles. Freckles (F) are dominant over no freckles (f). What are the chances that their children will have freckles?

• A woman is homozygous dominant for short fingers (SS). She marries a man who is heterozygous for short fingers (Ss). Will any of their children have long fingers (ss)? yes / no

Requires some deep thought…

Start by writing what you know!An allele for brown eyes B is dominant

over that for blue eyes b. A couple of whom one is brown-eyed and the other blue-eyed have eight children, all brown eyed. What would be the genetic make up of each parent in this regard? For each parent state whether they are

homozygous or heterozygous.

Here’s a tricky one…

• A blue-eyed man, both of whose parents were brown-eyed, marries a brown-eyed woman. They have one child who is blue-eyed. What are the genotypes of all the individuals mentioned?

• FYI: BROWN IS DOMINANT OVER BLUE, USE B’S JUST LIKE LAST PROBLEM

*Reproduction*Asexual

One parent = one form of the gene

Self-pollination results in a true

breed.

Sexual

2 parents

= variations in genes

Cross-pollination, conjugation or

fertilization results in a hybrid.

Mendel In Summary

• 1st Law of Dominance: A dominate trait masks or hide the expression of the other trait

• 2nd Law of Segregation: alleles segregate during gamete formation (meiosis)

• Offspring inherit 2 alleles for each trait

• Allele combinations are either homozygous or heterozygous, which influences expression.