Mendelian genetics
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Mendelian Genetics. Key Terms in Mendelian Genetics. Phenotype: observable traits Examples: brown eyes, yellow flower Genotype: actual alleles; describes the genetic characteristics The combination of alleles inherited Examples: BB, dd , Ff. Phenotype: brown eyes

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Mendelian Genetics

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Mendelian genetics

Mendelian Genetics


Key terms in mendelian genetics

Key Terms in Mendelian Genetics

  • Phenotype: observable traits

    • Examples: brown eyes, yellow flower

  • Genotype: actual alleles; describes the genetic characteristics

    • The combination of alleles inherited

    • Examples: BB, dd, Ff

Phenotype: brown eyes

This is the trait that is expressed!

Genotype: could be BB or Bb

This is the 2 alleles that were inherited from mother & father!


Key terms in mendelian genetics1

Key Terms in Mendelian Genetics

  • Homozygous: having 2 identical alleles for a trait

    • TT or tt

    • “homo” means “same”

  • Heterozygous: having 2 different alleles for a trait

    • Tt

    • “hetero” means “different”

Genotype: could be BB or Bb

This person could be homozygous dominant (BB) or heterozygous (Bb) for the eye color trait!


What color hair will kate middleton s prince william s baby have

What color hair will Kate Middleton’s & Prince William’s baby have?

  • We can use our knowledge of genetics & meiosis to make a prediction!


Meiosis

Meiosis

  • Remember!

    • During meiosis, our bodies make egg & sperm cells.

    • Egg & sperm cells contain ½ the amount of DNA as other cells. Why?


What color hair will kate middleton s prince william s baby have1

What color hair will Kate Middleton’s & Prince William’s baby have?

  • We now know that Prince William can only pass on the “b” allele.

  • Kate can pass on a “B” or a “b” to the baby.

  • To make our prediction, we have to draw a Punnett Square!


Punnett squares

Punnett Squares

  • Diagrams we can draw to help us predict the genotypes & phenotypes of offspring


Start with a blank box with 4 empty squares

Start with a blank box with 4 empty squares.

Add the genotype of each parent to the top & left side of the box (outside).

Father is on top & mother is on the left side.

b

b

B

b


Fill in the squares

Fill in the squares!

Remember that each parent only passes on 1 allele for the hair color gene!

b

b

B

Bb

Bb

bb

bb

b


Mendelian genetics

Remember, alleles are a version of a gene.


Mendelian genetics

What do the results of the Punnett square mean?

Kate and William will have 4 children

Two children will look like Kate, two will look like William

There is chance that a baby will have brown hair, and a chance that it will have blonde hair


Mendelian genetics

B=brown, b=blonde

What color hair will the offspring have if they inherit these alleles? Bb=?


Mendelian genetics

What are the chances that the baby will have blonde hair?

25%

75%

50%

No chance

50%


Mendelian genetics

3:1Phenotype = tall or shortThere’s a 75% chance the offspring will be tall & a 25% chance the offspring will be short!

What’s the phenotypic ratio in the offspring?

  • 1:2:1

  • 1:1:1:1

  • 3:1

  • What?!


Mendelian inheritance reflects the rules of probability

Mendelian inheritance reflects the rules of probability!

  • Mendel’s laws of segregation & independent assortment reflect the same laws of probability that apply to tossing coins or rolling dice.

    • The probability of tossing heads with a normal coin is 1/2.


When tossing a coin the outcome of 1 toss has no impact on the outcome of the next toss

When tossing a coin, the outcome of 1 toss has no impact on the outcome of the next toss!

  • Each toss is an independent event, just like the distribution of alleles into gametes.

Like a coin toss, each egg cell from a heterozygous parent has a ½ chance of carrying the dominant allele & ½ chance of carrying the recessive allele!

(The same odds apply to sperm.)


Law of segregation

Law of Segregation

This parent is carrying 2 alleles for a gene.

Genotype = Ss

During meiosis, the alleles are separated into 4 gametes.

Each gamete has a 50% chance of carrying S & a 50% chance of carrying s.


By the law of segregation the 2 alleles for a gene are packaged into separate gametes

By the law of segregation, the 2 alleles for a gene are packaged into separate gametes.

  • Here, the F1 hybrids all have purple flowers.


Mendelian genetics

When Mendel crossed the F1 plants together, the F2 generation included both purple & white flowered plants!

  • The white flowered allele was absent in the F1 plants, but reappeared in the F2 plants!

Based on a large sample size, Mendel recorded 705 purple plants & 224 white plants in the F2 generation.

What’s the ratio of purple to white plants?


Mendelian ratios

Mendelian Ratios

¼ of the F2 offspring will inherit 2 white flower alleles & produce white flowers. (p + p = pp)

½ of the F2 offspring will inherit 1 white flower allele & 1 purple flower allele & produce purple flowers.

(p + P = Pp)

¼ of the F2 offspring will inherit 2 purple flower alleles & produce purple flowers. (P + P = PP)

This represents a 3:1 ratio in the F2 generation!


What s the phenotypic ratio for these offspring

What’s the phenotypic ratio for these offspring?

  • 1:2:1

  • 3:1

  • 2:1

  • 1:1

1:1

Phenotype = yellow or green pea pod

The chances of the pea pods being yellow are equal to them becoming green!


The genotypic ratios

The Genotypic Ratios

50% Yy

0% YY

50% yy


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