Mendelian Inheritance

Biology In Focus Chapter 11AP Biology 2014

Ms. Eggers

Learning Objectives Define Mendel's law of segregation. Contrast dominant alleles with recessive alleles. Define genotype and phenotype. Explain how the law of probability applies to genetics. Describe how a Punnett square can be used to predict the results

of a genetic cross. Describe how a pedigree may be used to determine the mode of

inheritance of a genetic trait. List some well-known genetic disorders in humans. Explain the inheritance pattern of traits where more than two

alleles for the trait exist. Define incomplete dominance and incomplete penetrance. Define what is meant by pleiotropy. Describe the inheritance of polygenic traits. Explain X-linked inheritance and give some examples of X-linked

traits in humans.

Gregor Mendel (1822-1884)

Austrian monk Biology & math geek?

Yup! Explored heredity: the

passage of characters (a heritable feature that varies among individuals) from parents to offspring.

Looked at TONS of pea plants for a variety of different traits and came to conclusions about the transmission of characters: Mendel’s Principles of Heredity.

Mendel’s choice of pea plants as his experimental model was key

Pea plants reproduce quickly

The mating of plants could be precisely controlled

Peas demonstrate multiple traits that occur in two, distinct forms

Crossing pea

plants

Dichotomous traits in peas

The Monohybrid Cross Monohybrid: the two

plants crossed were both “true-breeding” but contrasting for one single trait.

True-breeding: all of the offspring display only one form of the trait.

Mating two true-breeding varieties is called hybridization.

What are the names of the various generations?

What did he conclude from the results of his monohybrid

crosses? First – in the F1 generation the white

flower disappeared. But it reappeared in the F2 generation! This gave Mendel the notion that hereditary factors remain discreet – and do not blend. He called the trait present in the F1 the dominant character and the trait that was disguised during the F1 the recessive trait.

Hereditary factor = gene

Genes come in variations

The variation of a gene is called an allele

Genes/alleles for the same trait are located in the same position on homologous chromosomes

What else did he conclude from the monohybrid cross?

Second – when he counted the number of purple flowers and white flowers in the F2 generation, he saw 705 purple and 224 white flowered plants. He consistently saw that the dominant and recessive traits appeared in a 3:1 ratio. Another way of saying this is that ¾ of the plants had purple flowers and ¼ of the plants had white flowers.

In total, he looked at over 21,000 hybrids pea plants for these

experiments!

Mendel knew his math…

Mendel took this information and said to himself, “Self. I know a little bit about probability. If each parent has 2 copies of the “hereditary factor” for that trait but they only give one copy to their offspring (so the offspring gets one from each parent), then it’s just the flip of a coin which copy the offspring gets from each parent… So…

Genetics? Or calculating the odds in Vegas?

If a parent has genotype Aa (is heterozygous), she can give either A or a to her offspring and its just a 50:50 chance – or ½.

Same for the other parent…

The probability that the offspring will get any combination is the product of the two separate probabilities (½ x ½ = ¼).

Vocabulary Heredity: the passing of characters from parents to

offspring Genetics: the study of heredity True-breeding: all of the offspring of a self-cross in

plants display only one form of the character P generation, F1 generation, F2 generation Alleles: different versions of a gene Dominant: the expressed form of a characteristic Recessive: the form of the trait not expressed

when the dominant allele is present Homozygous: having two of the same allele Heterozygous: having two different alleles Genotype: refers to the combination of alleles Phenotype: the physical appearance of a

characteristic

This handy picture is a Punnett Square – and it shows what

probability predicts.

Monohybrid cross ratios

The ratio of phenotypes in the F2 generation is 3:1 (or ¾ are the dominant phenotype and ¼ is the recessive phenotype).

The ratio of genotypes is 1:2:1 for AA:Aa:aa

Remember meiosis?

Mendel’s law of segregation

The two alleles for a character segregate (separate) when gametes are formed.

In other words, during meiosis I the homologous chromosomes separate so that each gamete only gets one copy of any chromosome/gene.

Dihybrid cross: two distinct traits at a

time

Dihybrid cross ratios Ratio of phenotypes is

9:3:3:1 of plants with both dominant traits: one dominant and the other recessive: one dominant and the other recessive the other way: and both recessive.

Again, probability shows why!

Think of each trait separately For round vs. wrinkled seeds… in the F2

generation ¾ offspring will be round and ¼ will be wrinkled.

For yellow vs. green seeds… in the F2 generation ¾ offspring will be round and ¼ will be wrinkled.

So the chances of getting both round and yellow (both dominant) is ¾ x ¾ = 9/16. The chances of getting round and green is ¾ x ¼ = 3/16. The chances of getting wrinkled and yellow is ¼ x ¾ = 3/16. And the chances of getting both green and wrinkled (both recessive) is ¼ x ¼ = 1/16.

This gives a ratio of 9:3:3:1.

Each trait is separate – Mendel’s law of independent assortment.

What if you have a purple pea plant?

Can you know if it’s AA or Aa? Not by looking…

But, you can do a test cross, which is crossing the unknown genotype with a pea plant that shows the recessive phenotype (and thus known genotype of aa).

Dihybrid test cross

Lucky Mendel got lucky. He chose the

perfect experimental model with which to work. Peas show numerous traits with discreet completely dominant and recessive phenotypes.

Inheritance patterns for most traits are usually more complex

Incomplete dominance

Snap dragons exhibit incomplete dominance of flower color

Incomplete dominance: when offspring have a phenotype that lies somewhere between the two phenotypes of the parents

The Pp offspring make less red pigment than the PP plants & thus appear pink

Codominance

The two alleles affect the phenotype in separate, distinguishable ways

Homework Explain how the inheritance of

the human disease Tay-sachs demonstrates distinct dominant-recessive phenotype, incomplete dominance AND codominance at various levels. (p. 216)

Multiple alleles – human blood types

Pleiotropy

When a gene has multiple phenotypic effects

Even more complicated – when multiple genes interact to confer a

given phenotype

Epistasis: the phenotypic expression of a gene at one locus alters than of a gene at a second locus

Lab coat color – the gene for pigment deposition (E/e) is epistatic to the gene that codes for black or brown pigment (B/b)

Polygenic inheritance

Quantitative characters vary in a population along a continuum – such as human height or human skin color

This usually indicates polygenic inheritance = multiple genes contributing to a single phenotype

Pedigree analysis Following human inheritance

through generations can be informative

And some human traits do show simple Mendelian inheritance

Inherited human disorders

Cystic fibrosis: lethal recessive disorder. 1:25 people of European descent are carriers for this recessive allele. Results in defective chloride transport channels

Achondroplasia: a dominant trait

Huntington’s disease: a dominantly inherited disorder that results in neurodegeneration and death

Sickle cell anemia