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Inheritance NotesUnderstandings:

Gene loci are said to be linked if on the same chromosome.

Unlinked genes segregate independently because of meiosis.

Variation can be discrete or continuous. The phenotypes of polygenic characteristics

tend to show continuous variation. Chi-squared tests are used to determine

whether the difference between an observed and expected frequency distribution is statistically significant.

Applications and Skills: Morgan’s discovery of non-Mendelian ratios in Drosophila. Completion and analysis of Punnett squares for dihybrid traits. Polygenic traits such as human height may also be influenced by environmental

factors. Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid

crosses involving unlinked autosomal genes. Identification of recombinants in crosses involving two linked genes. Use of chi-squared test on data from dihybrid crosses.

Notes about genes, alleles, loci, and Gregor Mendel:

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Notes on genotype, phenotype, homozygous, heterozygous, dominant, and recessive:

Complete, Incomplete, and Codominance:

Complete dominance-

Example:

Incomplete dominance-

Example:

Codominance-

Example:

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Codominance in multiple allele systems:

Example of a blood group cross: (indicate P generation and F1 generation)

Punnett Squares and Test Crosses:

Punnett Squares- can be used to show how the alleles of parents are split between their gametes and how new combinations of alleles can show up in offspring.

Example: BB x Bb

Test cross- A cross between an unknown-genotype organism and a known homozygous recessive organism. To figure out the genotype of the unknown individual.

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Ratio for a cross between two heterozygous individuals, for a single trait =

Monohybrid and Dihybrid Crosses:

Monohybrid cross- takes into account _______ genetic trait.

Example:

Dihybrid cross- takes into account __________ genetic traits.

Example:

Number of different gamete combinations=

FOIL=

Genotype:

Foil Method:

Gametes:

Practice: Dihybrid Cross, unlinked autosomal genes

In sheep, the allele for black wool (B) is dominant over the allele for white wool (b).

Similarly, the allele for horns (H) is dominant over the allele for being hornless (h).

Pure breeding horned sheep with black wool were crossed with pure breeding hornless sheep with white wool.

(a) State the genotype and the

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Practice: Dihybrid Cross, unlinked autosomal genes

Sex-linked traits- have their loci on a sex chromosome.

Example:

Autosomal disorders- have their loci on one of the 22 (humans) autosomal chromosomes.

Example:

In sheep, the allele for black wool (B) is dominant over the allele for white wool (b).

Similarly, the allele for horns (H) is dominant over the allele for being hornless (h).

Pure breeding horned sheep with black wool were crossed with pure breeding hornless sheep with white wool.

(a) State the genotype and the

In cats, the allele for grey fur (G) is dominant over the allele for beige fur (g).

The allele for a solid coat (S) is dominant over the allele for a striped coat (s).

A pure breeding solid, beige cat is crossed with a pure breeding striped, grey cat.

(a) State the genotype and the phenotype of the F1 individuals produced as a result of this cross.(b) Calculate the phenotypes resulting from a cross between a pure breeding solid, beige cat and an F1 offspring.

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Practice: Sex-linked Traits

Pedigree Charts:Inheritance of white coat color in mice:

Linked and Unlinked Genes:

Law of Independent Assortment-

1. Males are represented as ______________, females are represented as circles.

2. Shaded symbols mean that an individual is affected by the condition, unshaded is unaffected.

3. A horizontal line between man and woman indicates _____________.

4. Generations are labeled with roman numerals and numbered according to age (oldest on the left).

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Linked Genes-

*

*

Morgan’s Discovery of Non-Mendelian Ratios in Drosophila:

Experiment:

Two Key Proposals:1.

2.Additional gene linkage notes:

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Determining Significance: Chi-Squared Tests

Purpose of Chi-Squared tests-

Chi-squared formula =

p-value-

p ≤ 0.05 =

p > 0.05 =

HO=

HA=

Steps to doing a chi-squared problem:1. Identify the hypothesis (null and alternative)

2. Construct a table of frequencies (observed v. expected)

3. Apply the chi-squared formula

4. Determine the degree of freedom (df)

5. Identify the p-value (should be ≤ 0.05)

Practice: Chi-squared of pea texture and color

The trait for smooth peas (R) is

dominant over wrinkled peas (r)

and yellow pea colour (Y) is

dominant to green (y)

A dihybrid cross between two

heterozygous pea plants is

performed (RrYy × RrYy)

The following phenotypic

frequencies are observed:

701 smooth yellow peas

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Step 1: Null and Alternative Hypotheses

Null Hypothesis (HO):

Alternative Hypothesis (HA):

Step 2: Table of FrequenciesSmooth Yellow

Smooth Green

Wrinkled Yellow

Wrinkled Green

Total

Observed (O)

Expected (E)

Step 3: Chi-Squared Formula

Smooth Yellow Smooth Green Wrinkled Yellow Wrinkled Green

Observed (O)

Expected (E)

(O-E)

The trait for smooth peas (R) is

dominant over wrinkled peas (r)

and yellow pea colour (Y) is

dominant to green (y)

A dihybrid cross between two

heterozygous pea plants is

performed (RrYy × RrYy)

The following phenotypic

frequencies are observed:

701 smooth yellow peas

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(O−E )2

E

χ2=¿

Step 4: Degrees of Freedom (df)

(number of phenotypes) – 1 = df

Step 5: The p Value

A value is considered significant is there is less than a 5% chance that the results are due to random chance.

Conclusion:

Practice: Chi-Squared in Sweet Pea Plants

In sweet pea plants, the trait for purple flowers (P) is dominant to the trait for red flowers (p).

Similarly, the trait for long pollen (L) is dominant to the trait for round pollen (l).

Two heterozygotes are crossed, yielding the following frequencies for the F1 generation: 296 purple, long plants ; 19 purple, round plants ; 27 red, long plants ; 85 red, round plants

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Activity: Use the chi-squared test to determine if these results are due to independent assortment.

Polygenic inheritance- two or more genes influencing the expression of one trait. It is believed that most human traits are too complex and show too many combinations to be determined by one gene.

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Continuous variation (polygenic)- When an array of possible phenotypes can be produced. Influenced by environmental factors.

Examples:

Discontinuous variation (monogenic)- Individuals fall into distinct categories. Such data is called discrete (or categorical) data. “Either or traits”

Examples:

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