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Today: Onward through Mendelian Genetics and Exceptions Adding Chromosomes to the Story. Testing Mendel’s Law of Segregation: The Punnett Square. The Punnett Square for Mendel’s Experiments:. What will the F1 Generation look like? The F2 Generation?.

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Presentation Transcript
slide1
Today:
  • Onward through Mendelian Genetics and Exceptions
  • Adding Chromosomes to the Story
slide3
The Punnett Square for Mendel’s Experiments:

What will the F1 Generation look like? The F2 Generation?

slide6
Understanding the predicted results of a PUNNETT SQUARE, allows for a TESTCROSS

What’s my phenotype? My genotype?

slide8
Sickle Cell Disease Questions:
  • Two individuals who are heterozygous at the Sickle Cell locus have four children together. One of the children is affected with the disorder. Based on this information, is the sickle cell trait dominant or recessive?
slide9
Sickle Cell Disease Questions:

2. If the affected offspring has a child with an unaffected individual (who does not carry the sickle allele), what is the probability that any given child will be unaffected? Be a carrier? Be affected?

slide10
An Aside: Unusual Gene Frequencies!?

What do you notice? What does this suggest?

slide11
Mendelian Genetics- Example 2:

Cystic Fibrosis is also an Autosomal Recessive Trait with Unusual Gene Frequencies

  • If two carriers of the cystic fibrosis trait have children, what is the probability that their first child will be affected?
  • It they eventually have three children, what is the probability that all three will be affected?
dependent assortment
Dependent Assortment?

Mendel’s Next Question: What happens in a dihybrid cross?

What would the outcome look like if it’s dependent assortment??

what mendel sees
What Mendel Sees:

So is it dependent assortment??

mendel s contributions
Mendel’s Contributions

Law #1: Segregation

Law #2: Independent Assortment

practice problem monohybrid cross
Practice Problem: Monohybrid Cross

In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?

  • None B. ¼
  • C. ½ D. ¾
  • E. All
practice problem dihybrid cross
Practice Problem: Dihybrid Cross

In a dihybrid cross, AaBb x AaBb, what fraction of the offspring will be homozygous for both recessive traits?

  • 1/16 B. 1/8
  • C. 3/16 D. 1/4
  • E. 3/4
slide18
Complication #1: (Mendel was lucky!)

INCOMPLETE DOMINANCE

Heterozygotes have a unique phenotype, between that of the homozygous dominant or recessive parents.

Note: This is not blended inheritance!

Why?

slide20
Another Exception:Codominance
  • In codominance, both alleles affect the phenotype in separate, distinguishable ways.
  • Example:
  • Human blood groups M, N, and MN
  • Group MN produce both antigens on the surface of blood cells
slide21
Another Exception:Codominance

Example:

Tay-Sachs disease-

Heterozygous individuals produce both functional, and dysfunctional enzymes. organismal level = recessive, biological level = codominant.

A section of the brain of a Tay Sachs child. The empty vacuoles are lysosomes that had been filled with glycolipid until extracted with alcohol in preparing the tissue.

slide22
Three Important Points about Dominant/Recessive Traits:
  • They range from complete dominance  incomplete dominance  codominance. (can be a subtle distinction!)
  • They reflect mechanisms through which specific alleles are expressed in the phenotype (i.e. this is not one allele subduing another at the DNA level)
  • They’re not related to the abundance of an allele within a population!
slide23
Codominance Practice Question:
  • The Palomino horse is a hybrid exhibiting a golden color with lighter mane and tail.
  • A pair of codominant alleles (D1 and D2) control the inheritance of these coat colors.
  • Genotypes homozygous for the D1 allele are chestnut-colored (reddish), heterozygous genotypes are Palamino-colored, and genotypes homozygous for the D2 allele are almost white and called cremello.
slide24
Codominance Practice Question:
  • 1. From the matings between two Palaminos, determine the expected Palamino : non-Palamino ratio among the offspring.
  • What percentage of the non-Palamino offspring in part 1 will breed true?
  • 3. What kind of mating will produce only Palaminos?
slide25
From the matings between two Palaminos, determine the expected Palamino : non-Palamino ratio among the offspring.
  • P = D1D2 x D1D2F1 = 1/4 D1D1 + 1/2 D1D2 + 1/4 D2D2Ratio 1 Palomino : 1 non-Palomino
  • What percentage of the non-Palamino
  • offspring in part (a) will breed true?
  • all non-Palomino offspring will breed true
  • (c) What kind of mating will produce only Palaminos?
  • chestnut-colored x cremello will produce all Palominos
slide28
Practice Question:

Paternity testing

Scenario : Suppose mother is Type A, baby is Type B.

Consider these three putative fathers: can any be the actual father?

#1 (Type A): Yes or No? #2 (Type B): Yes or No? #3 (Type O): Yes or No?

further complications pleiotropy
Further Complications: Pleiotropy

Most genes have multiple phenotypic effects!

further complications pleiotropy1
Further Complications: Pleiotropy

No production of melanocytes during development causes:

1. White fur color

and

2. Inability to transmit electrical signals to brain from hair cells in the ear.

slide31
More Complications:

EPISTASIS

Example:

The “color gene”, C, allows pigment to be deposited in hair. When lacking, a mouse is albino, regardless of its genotype at the other locus.

epistasis and lab pups
Epistasis and Lab Pups

Coat color in labradors is determined by 2 genes, a pigment gene (B), and a pigment delivery gene (E).

Black is dominant to Brown, so Heterozygotes (Bb) are black. The delivery gene is also dominant, so EE or Ee individuals both express their pigments. Only ee individuals are yellow.

slide33
Epistasis and Lab Pups

Your Question:

If I cross a Brown Lab (bbEe) with a Black Lab (BbEe), can I expect any yellow puppies?

If so, what proportion of the pups would I expect to be yellow?

there s more polygenic inheritance
There’s more… Polygenic Inheritance

This results in a broad norm of reaction

slide35
Other Issues: Environmental Effects on Phenotype

Many factors, both genetic and environmental, influence the phenotype.

slide38
Similarities between the behavior of chromosomes and Mendel’s “factors”:
  • Chromosomes and genes are both present in paired in diploid cells
  • Homologous chromosomes separate and alleles segregate during meiosis
  • Fertilization restores the paired conditions for both chromosomes and genes
slide39
Similarities between the behavior of chromosomes and Mendel’s “factors”:

In 1902 the Chromosome Theory of Inheritance was proposed. In states that Mendelian genes have specific loci on chromosomes, and these chromosomes undergo segregation and independent assortment.

slide40
Correlating the results of Mendel’s dihybrid crosses with the behavior of chromosomes during meiosis
slide41
Thomas Hunt Morgan’s contributions: Fruit Fly Genetics
  • Single mating produces 100+ offspring
  • A new generation can be bred every two weeks
  • Only four pairs of chromosomes- 3 pairs of autosomes, 1 pair sex chromosomes (XX and XY)
slide42
Unlike Mendel, Morgan does not have access to true-breeding strains.

He breeds flies for a year, looking for distinct varieties.

He discovers a male fly with white eyes, instead of red.

In Drosophila,

red eyes = Wild type (the most common phenotype in a natural population)

white eyes = a Mutant Phenotype.

morgan s results
Morgan’s Results:

First Experiment:

Morgan crosses a red-eyed female with a white-eyed male. ALL the offspring have red eyes.

How would Mendel explain these results?? What would Mendel do next??

morgan s results1
Morgan’s Results:

Next Experiment:

Morgan crosses two of the red-eyed F1 flies with each other.

What should he see if Mendel is correct??

morgan s results2
Morgan’s Results:

He DOES find a 3:1 ratio, but ALL the white-eyed flies are male!!

Was Mendel wrong?? What happened?!?

slide47
Sex Determination Happens in a Variety of Ways

Sex chromosomes (especially the X chromosome) carry genes for many other characters.

In humans, the term “sex-linked” generally refers to genes on the X chromosome.

slide48
The Transmission of SEX-LINKED RECESSIVE Traits

In this diagram “A” represents a dominant allele carried on the X chromosome; “a” represents the recessive allele. White boxes indicate unaffected individuals, light-colored boxes are carriers, and dark-colored boxes are affected individuals. Note that both males and females are affected by sex-linked disorders!

slide50
In females, one X chromosome is inactivated (at random) and condenses into a compact Barr body along the inside of the nuclear envelope. Most genes on this X chromosome are not expressed.

Because it is random which X chromosome forms the Barr body during development, females are Mosaics of the two cell types.

slide51
In females, one X chromosome is inactivated (at random) and condenses into a compact Barr body along the inside of the nuclear envelope. Most genes on this X chromosome are not expressed.

Because it is random which X chromosome forms the Barr body during development, females are Mosaics of the two cell types.

practice question sex linked chromosomal inheritance
Practice Question: Sex-Linked Chromosomal Inheritance
  • If you see the number 74, then you do not have red-green color blindness. If you see the number 21, you are color blind to some extent. A totally color-blind person will not be able to see any of the numbers.
practice questions sex linked chromosomal inheritance
Practice Questions: Sex-Linked Chromosomal Inheritance
  • If a color blind man has children with a “wild-type” woman, what are the chances that a daughter of theirs will be colorblind?
  • What are the chances that their son will be colorblind?
  • Can females be colorblind? What would the genotype of the parents have to be?
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