Inheritance3.4 & 10.2

Understandings● 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.

Key Vocabulary To Watch For

● Loci● Linked genes● Unlinked genes● Independent assortment● Discrete (discontinuous)

variation● Continuous variation● Phenotype● Genotype● Polygenic characteristics

● Chi-squared test● Statistically significant● Punnett squares● Dihybrid● Monohybrid● Autosomal genes● Sex genes● Recombinants

Genes, Alleles, Loci, and Gregor Mendel Gregor Mendel (1865): Published his results on experiments involving pea plant traits.

★ “Factors” (DNA’s role in heredity was not yet discovered).

★ What were some questions that he may have asked regarding his experiments?

Alleles: Varying forms of a gene that occur at a specific locus. Most genes exist in many forms.● Our chromosomes have 2 alleles for each

gene, one from our mother and the other from our father.

● Example: flower color (dominant allele vs. recessive allele)○ Purple ○ White

Genotype, Phenotype, Homozygous and Heterozygous, Dominant and Recessive

Genotype: The symbolic representation of the pair of alleles possessed by an organism, typically represented by two letters.

● (Bb, bb, BB, Hh, etc.)

Phenotype: The physical manifestation of the genotype. The characteristics or traits of an organism.

● (Five fingers on each hand, color blindness, purple flowers, white flowers, etc.)

Homozygous: Having two identical alleles of a gene.

Heterozygous: Having two different alleles of a gene.

Dominant allele: An allele that has the same effect on the phenotype whether is it paired with the same allele or not. Always expressed in the phenotype!

Recessive allele: An allele that has an effect on the phenotype only when in the homozygous state.

Practice: For each genotype, indicate whether it is heterozygous or homozygous. Then, determine the phenotype.

1. Purple flowers are dominant to white flowers:a. PP b. Ppc. pp

2. Brown eyes are dominant to blue eyes:a. BBb. Bbc. bb

Complete Dominance and CodominanceComplete Dominance-

● The dominant allele will mask the recessive allele when in a heterozygous state.

● Homozygous dominant and heterozygous forms will be phenotypically indistinguishable.

● The recessive allele will only be expressed in the phenotype when in a homozygous state.

Codominance-

● Both both alleles are expressed equally in the heterozygous individual’s phenotype.

Practice: Codominance in Multiple Allele SystemsRead this passage then fill in the table to the right.

The human ABO blood group system is a multiple allele system involving the codominant alleles: IA, IB, and the recessive allele i.

The four common blood groups in humans are determined by the alleles: IA, IB, and i are A, B, AB, and O. The ABO antigens consist of sugars attached to the surface of red blood cells. The alleles code for enzymes (proteins) that join these sugars together. The allele i produces a non-functioning enzyme that is unable to make any changes to the basic antigen (sugar) molecule. The other two alleles (IA, IB) are codominant and are expressed equally. They each produce a different functional enzyme that adds a different, specific sugar to the basic sugar molecule. The blood group A and B antigens are able to react with antibodies present in the blood of other people so blood must always be matched for transfusion.

Practice: Use the information about blood groups to complete crosses 2 and 3.

Summary of ABO Blood Groups

Punnett Squares and Test CrossesPunnett Square: 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. Used in monohybrid crosses.

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

Monohybrid and Dihybrid CrossesMonohybrid Cross: Takes into account only one genetic trait.

Dihybrid Cross: Takes into account two genetic traits.

Completion and analysis of punnett squares for dihybrid traitsA dihybrid cross determines the genotypic and phenotypic combinations of offspring for two particular genes that are unlinked

■ Because there are two genes, each with two alleles, there can be up to four different gamete combinations

The easiest way to work out potential gamete combinations in a dihybrid cross is to use the FOIL method:

■ FOIL = First / Outside / Inside / Last

Practice: Complete the dihybrid cross and figure out the ratio of phenotypes and genotypes that result from the cross.

Practice: Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving 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 phenotype of the F1 individuals produced as a result of this cross. (b) Two F1 offspring were mated together. Calculate the expected ratio of phenotypes in the F2 generation.

Practice: Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.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.

Autosomal and Sex Chromosomes

Sex-linked traits: Have their loci on a sex chromosome.● Color blindness● Haemophilia

Autosomal Disorders: If a trait or gene is said to be autosomal, its locus is on one of the 22 autosomal chromosomes.● Huntington’s disease● Cystic fibrosis

Genetic DiseasesAn autosomal recessive genetic disease will only occur if both alleles are faulty

■ Heterozygous individuals will possess one copy of the faulty allele but not develop disease symptoms (they are carriers)

■ An example of an autosomal recessive genetic disease is cystic fibrosis

.

An autosomal dominant genetic disease only requires one copy of a faulty allele to cause the disorder

■ Homozygous dominant and heterozygous individuals will both develop the full range of disease symptoms

■ An example of an autosomal dominant genetic disease is Huntington’s disease

If a genetic disease is caused by codominant alleles it will also only require one copy of the faulty allele to occur

■ However, heterozygous individuals will have milder symptoms due to the moderating influence of a normal allele

■ An example of a genetic disease that displays co-dominance is sickle cell anaemia

Practice: Sex-Linked TraitsHemophilia is an inherited genetic disorder linked to the X-chromosome that results in ineffective blood clotting when a blood vessel is damaged. The most common type, hemophilia A, occurs in 1 in 5,000 male births. Any male who carries the gene will express the phenotype. Hemophilia is extremely rare in women.

A couple wish to have children. The woman knows she is a carrier for hemophilia. The man is not a hemophiliac. Use the notation Xh for hemophilia and XH for the dominant allele to complete the diagram on the right including the parent genotypes, gametes, and possible fertilizations. Write the genotypes and phenotypes in the table.

Practice: Sex-Linked TraitsAs a genetic counselor you are presented with a married couple where one of them has a family history of rickets, an X-linked condition shown in the alleles, genotypes, and phenotypes to the right.

The husband is affected by this disease and the wife is normal. The couple, who are thinking of starting a family, would like to know what their chances are of having a child born with this condition. They would also like to know what the possibilities are of having an affected boy or affected girl. Use the symbols to complete the diagram and determine the probabilities (expressed as a proportion or percentage).

Analysis of pedigree charts to deduce the patterns of inheritance of genetic diseases

● Males are represented as squares, females are represented as circles.

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

● A horizontal line between man and woman indicates mating.

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

Practice: Pedigree

Unlinked genes segregate independently as a results of meiosis

Law of independent assortment: alleles separate independently during gamete formation, and traits are passed on to offspring independently of one another (true for unlinked genes).

Linked Genes/Linkage Groups and Independent Assortment

Linked Genes: When two genes are close together on a chromosome. THEY DO NOT ASSORT INDEPENDENTLY, they are said to be “linked”.

● Do not follow expected Mendelian ratios.

● Can be detected by observing phenotypic ratios in the offspring.

Law of Independent Assortment: Allele pairs separate independently during gamete formation, and traits passed on to offspring independently from each other. (true only for unlinked genes).

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

Practice: Inheritance of Linked Genes Worksheet

Morgan’s discovery of non-Mendelian ratios in Drosophila● Linked genes in Drosophila melanogaster

(fruit fly).● Cross-breeding red-eyed wild types with

white-eyed mutants, he discovered a clear sex bias in the phenotypic distribution.

● All female offspring of a red-eyed male were red-eyed, whereas all male offspring of a white-eyed female were also white-eyed

● Morgan described this distribution as 'sex-limited’ inheritance and inferred it was caused by the gene for eye colour being located on a sex chromosome (i.e. X-linked)

Based on this data, Morgan made two key proposals:

1. The alleles for these traits were located on a shared chromosome (gene linkage) and hence did not independently assort.

2. Linked alleles could be uncoupled via recombination (crossing over) to create alternative phenotypic combinations, but these new phenotypes would occur at a much lower frequency.

Recombinants

The term is used to describe both the new chromosome and the resulting organism.

New traits that neither of the parents have.

Recombinant phenotypes can be identified using a testcross (cross a homozygous recessive for both traits)

Practice: Recombination and Dihybrid Inheritance Worksheet

Fruit Fly Activity

Testing for Gene Linkage

Determining Significance: Chi-Squared TestsTest used to determine whether the difference between an observed result and an expected result is statistically significant.

P-value: Probability of significance

p ≤ 0.05 ('p is less than or equal to 0.05'), reject H0 in favour of H1

p > 0.05 ('p is greater than 0.05'), fail to reject H0 Ho= Null Hypothesis: Assumes no difference, or relationship between variables.

Ha= Alternative Hypothesis: Assumes a difference, or relationship between variables.

Frequency distribution of potential phenotypes

Steps to a Chi-Squared Test

1. Identify the hypotheses (null and alternative)2. Construct a table of frequencies (observed v.

expected)3. Apply the chi-squared formula4. Determine the degree of freedom (df)5. Identify the p-value (should be ≤ 0.05)

ExampleThe 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 ■ 204 smooth green peas ■ 243 wrinkled yellow peas■ 68 wrinkled green peas

Step 1: Null and Alternative Hypotheses

■ Null hypothesis (HO ): There is no significant difference between observed and expected frequencies (i.e. genes are unlinked).

■ Alternative hypothesis (HA ): There is a significant difference between observed and expected frequencies (i.e. genes are linked).

Step 2: Table of Frequencies

Step 3: Chi-Squared Formula

SUM

Observed Frequency

Expected Frequency

Step 4: Degrees of Freedom (df)

df = (m – 1) (n – 1)

Where: m = number of rows ; n = number of columns

Degree of freedom should be: (number of phenotypes-1)

Step 5: The p Value

A value is considered significant if there is less than a 5% probability (p < 0.05) the results are attributable to chance.

Practice: Chi-Squared in Sweet Pea PlantsIn 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

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

More PracticeExpected ratio Observed # (O) Expected # (E) O-E (O-E)2 (O-E)2/E

Total

A genetics engineer was attempting to cross a tiger and a cheetah. She predicted a phenotypic outcome of the traits she was observing to be in the following ratio 4 stripes only: 3 spots only: 9 both stripes and spots. When the cross was performed and she counted the individuals she found 50 with stripes only, 41 with spots only and 85 with both. According to the Chi-square test, did she get the predicted outcome?

Polygenic Inheritance, Continuous, and Discrete VariationPolygenic 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.

Continuous Variation (polygenic): When an array of possible phenotypes can be produced. Influenced by environmental factors.● Skin color● Height● Body shape● Intellectual aptitude

Discontinuous variation (monogenic): Individuals fall into distinct categories. Such data is called discrete (or categorical) data. “Either or traits”● Blood type● Widow’s peak● Rolling tongue● Ear lobes

Maize Grain Colour

● Controlled by 3 gene loci.● Ranges from white to dark

red, depending on amount of pigment expressed.

● Each gene has two alleles, which either code for red pigment or white pigment.

● Overall pattern shows continuous variation

Polygenic characteristics such as human height may be influenced by environmental factors

■ Human height is controlled by multiple genes (polygenic), resulting in a bell-shaped spectrum of potential phenotypes

■ Environmental factors such as diet and health (disease) can further influence an individual human’s height

GMO: Identification of Genetically Modified Foods Using PCR