Mendelian Genetics

GeneticsStudy of heredity, or the

passing on of characteristics from parent to offspring.

**Reproduction Reproduction ReviewReview**

Asexual

One parent = genetically identical

Results in a true breed.

Sexual

Two parents

= genetic variations

Results in a hybrid.

Other concepts for reviewOther concepts for review

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

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

How was genetics How was genetics discovered?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?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…

But Mendel controlled the pollination of two true breed pea plants (see picture at right).

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

crossed with a true breed green pea plant?

What happened to the What happened to the yellow?yellow?

Did yellow disappear or is green simply dominant?

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

Yellow did not disappear; green is dominant

**What did he realize?What did he realize?**

1. There is some hidden factor that controls inheritance. It’s called a gene. Remember genes are segments of a chromosome and we have pairs of homologous chromosomes: one maternal and one paternal.

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 version of the trait will mask the expression of the recessive version.

**Mendel also realizedMendel also realized**

Alleles are carried on opposite homologous chromosomes (shown at right).

**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 (i.e. homologous pair). There are 3 possible allelic combinations that can be inherited. These are known as genotypes. Genotypes are either homozygous (both alleles are identical; AA or aa) or it is heterozygous (alleles are different; Aa). Genotypes are expressed as phenotypes. These are the observable characteristics.

See the table of genotypes and corresponding phenotypes on the next slide.

**Mendel also realizedMendel also realized**

Genotype(allele type or “ingredients”)

Phenotype(expression or “cake”)

TT(homozygous=same)

Tall(Dominant)

Tt(heterozygous=diff.)

Tall(Dominant)

tt(homozygous)

Short(Recessive)

5. Gametes play a role in genetics. The two alleles of a person’s genotype segregate into separate gametes during meiosis so each sex cell only carries one allele for each trait. This is known as the Law of Segregation.

If each organism has two alleles for each trait, there are four possible genetic recombinations that can result from the fertilization of the two parents’ gametes. Thus, genetic

outcomes of offspring can be predicted mathematically.

Watch the segregation of A, a, B, b genes by clicking here

**Mendel also realizedMendel also realized**

**Predicting genetic Predicting genetic outcomesoutcomes**

A 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 a 50% probability of inheriting from either gamete).

**Predicting genetic Predicting genetic outcomesoutcomes**

1. One parent’s genotype is segregated on top of the square: one allele over each column

2. The other parent’s genotype is segregated on the side of the square: one allele beside each row

**Predicting genetic Predicting genetic outcomesoutcomes**

3. Then each allele from the top is distributed down into each box beneath it. In the example at the right B is distributed down each column.

4. And each allele from the side is distributed into each box to the right. In the example at right b is distributed across.

*Predicting genetic outcomes*

Probability of inheriting traits:

•Of the four gametes produced by meiosis (example shown at right), 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% (2 out of 4) chance that a gamete with A may be fertilized and a 50% chance that a gamete with a may be fertilized. Likewise for the B gene.

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 green hybrids (Gg)

What did What did Mendel’s Mendel’s cross look cross look like?like?

**Predicted OutcomesPredicted Outcomes**

What probable phenotypes are produced?

= Ratio of green to yellow

What probable genotypes are produced?

= Ratio of GG:Gg:gg

How to solve a genetics problem

1.1. Write out the genotype key!!!Write out the genotype key!!!

GG – green, Gg – green, gg – yellow

2. Write out parent genotypes of test cross

3. Draw Punnett square

4. Segregate each parent’s alleles

5. Complete the square

6. Compare genotype & phenotype ratios

Try this one…

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

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

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

Solution #1

• TT, Tt, tt (see Punnett Square)

T t

T TT Tt

t Tt tt

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

Solution #2

• 3 out of 4 chance or 75% (see Punnett Square)

F f

F FF Ff

f Ff ff

Solution #3

• No – only ss genotypes are recessive (long fingers) and no offspring have this genotypeS S

S SS SS

s Ss Ss

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.

Solution #4

• One parent must be bb because she is blue-eyed

• The other parentmust be BB ifnone of their kidsare blue-eyed

B B

b Bb Bb

b Bb Bb

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 PROBLEMO

Solution #5

• Blue-eyed man = bb• One blue-eyed child = bb• Then, brown-eyed woman

must be Bb to have a blue-eyed child

• The father’s parents,who are brown-eyed,must both be Bb

b bBb bb

B bBb bb

* * Finally Mendel also observed Finally Mendel also observed **

Of an organism’s many different traits, many genetic combinations were observed. For instance, some pea plants had white flowers, round seeds, yellow pods while others had white flowers, wrinkled seeds, yellow pods. Others included: white, round, green; white, wrinkled, green; purple, round, yellow; purple, wrinkled, green…

This showed that versions of different traits did not influence one another, which can only be explained the Law of Independent Assortment: alleles for different traits assort separately and independently of one another.

Mendel In SummaryMendel In Summary

• 1st Law of Dominance: form of a trait masks the expression of the other form

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

• 3rd Law of Independent Assortment: alleles for different traits do not influence the inheritance of one another.

Lesson SummaryLesson Summary

One father of genetics

Two alleles for every trait

Three laws of inheritance

Four possible genetic outcomes (Monohybrid Punnett Square)