6 1 Beyond Mendels Observations of Inheritance SBI

6. 1 -Beyond Mendel’s Observations of Inheritance SBI 3 U

Beyond Gregor Mendel… • Mendel’s experiments were simple and provided a basis for an understanding of inheritance • However, scientists realized that patterns of inheritance are more complicated than what he proposed. • Dominance and recessiveness are not the only patterns.

1)Incomplete Dominance When two true-breeding plants are hybridized, all of the offspring are heterozygous and have pink flowers

1)Incomplete Dominance • A heterozygote exhibits a phenotype that is somewhere in between the dominant and recessive phenotype ie. Pink flower *NOTE: Superscripts are used when the pattern of inheritance is incomplete dominance

1)Incomplete Dominance Example: Flowers P generation: True breeding: white and red F 1 generation: 100% of offspring appear pink F 2 generation: 25% red, 50% pink, 25% white (1: 2: 1 ratio)

2)Codominance • Both alleles are fully expressed. • The offspring contains spots which express both dominant alleles. *NOTE: Roan cow, product of mating between a red cow and a white cow. • For example, the roan cow. The red and white hairs can be found in patches are spread through the body.

*Note: Codominance and Incomplete Dominance are NOT the same thing. WHY?

Example of Codominance: Sickle Cell Anemia • Often referred to as autosomal recessive • irregular shaped red blood cells cannot transport oxygen as efficiently as normal red blood cells • Heterozygotes who carry the trait have some normal and some sickled red blood cells. Hb. A = normal hemoglobin Hb. S = sickle cell anemia

Codominance : Sickle Cell Anemia Hb. A Hb. S Hb. S Homozygous (Hb. A ) = normal Homozygous (Hb. S )= sickle cell Heterozygous = have some normal red blood cell and some abnormal (heterozygous advantage= resistant to malaria) Both offspring will have a mixture of normal and sickle cell RBC.

Test your Understanding…Problem 1: A plant that produces white flowers is crossed with a plant that produces purple flowers. a) If the pattern of inheritance is incomplete dominance what would the phenotype(s) of the F 1 and F 2 offspring be? Use a Punnett square to support your answer.

Test your Understanding…Problem 2 A plant that produces white flowers is crossed with a plant that produces purple flowers. a) Describe the phenotype(s) of the F 1 and F 2 offspring if the inheritance pattern is codominant. Use a Punnett square to support your answer.

3) Multiple Alleles • Many traits that exist in population result in more than two alleles for one gene. • Example: Human blood types ABO blood type group in humans have multiple alleles for one gene (A, B, O). The allele determines which antigen will be found on the surface of the red blood cell (RBC).

Different combinations of the three I alleles result in 4 different blood types: type A type B type AB type O

3)Multiple Alleles: Blood Types • The gene for blood type is designated “I” with superscripts. • 3 different alleles: ▫ IA (blood type A) ▫ IB (blood type B) ▫ i (blood type O) *recessive • Phenotype and genotype: ▫ ▫ type A (IAIA , IA i) B (IBIB , IBi) AB (IAIB ) O (ii homozygous)

Check Your Understanding… A)If a man has type O blood and a woman has type B blood what possible blood types could their children have? Draw a Punnett square to show every possibility.

Check Your Understanding… B)If this couple has six children, all with type B blood, what would you infer about the woman’s genotype?

3) Multiple Alleles Cont’d • There are 4 alleles that control the gene of rabbit coat colour (in order of dominance) ▫ ▫ Agouti ( C) Chinchilla (cch) Himalayan (ch) Albino (c)

Check Your Understanding… A chinchilla rabbit with genotype cch ch is crossed with a Himalayan rabbit with genotype ch c. What is the expected ratio of phenotypes among the offspring? Use a Punnett square.

Environmental Effect on Inheritance • Some genes are affected by temperature ▫ Example: Himalayan rabbits have darker colours on the cooler parts for their bodies. ▫ The colour is from a gene that is only active at lower temperatures *Note: The dark ears, nose, feet, and tails of Himalayan rabbits are thought to be caused by lower body temperature in these areas.

4) Polygenic Inheritance • There are traits that exhibit continuous variation where phenotypes vary gradually from one extreme to another. Examples: height, skin colour, kernel colour in wheat, and ear length in corn. These are generally controlled by more than one gene and are called polygenic traits.

AABBCC – person is very dark aabbcc – person is very fair Aa. Bb. Cc – person has an intermediate shade Skin colour in humans is controlled by at least three separately inherited genes. Each gene has its own separate alleles dominant A, B, C and recessive a, b, c

HOMEWORK • Read and make notes 6. 1 • Complete pg 247 #1, 2, 5, 6, 8, 9