Unit 4: Heredity and Development 4-2 Mendelian Genetics

Important Vocabulary  True-breeding-  Hybridization-  P generation-  F 1 generation-  F 2 generation-  Homozygous-  Heterozygous-  Genotype-  Phenotype-

Mendel’s Conclusions  Alternative versions of genes (alleles) account for variations in inherited characters  Gene  Allele

Dominant and Recessive Alleles  If the two alleles at a locus differ, then one, the dominant allele determines the organism’s appearance, the other the recessive allele has no noticeable effect

The Law of Segregation  Alleles segregated during gamete formation and end up in different gametes  Homozygous  True breeding  Two of the same alleles  100% of gametes will get the same allele  Heterozygous  One dominant and one recessive allele  50% of offspring will get the dominant allele  50% will get the recessive allele

 For each trait, an organism inherits two alleles, one from each parent

The Law of Independent Assortment  Each pair of alleles segregates independently of other alleles during gamete formation  NOTE: only applies to alleles located on separate, nonhomologous chromosomes.

Three Rules of Probability  Past Events never effect future outcomes.  A woman has 13 children-all of them boys! She is pregnant with a 14 th child. What is the probability that she will have a girl?

The Multiplication Rule  The probability of two separate events occurring simultaneously is the multiple of their separate probabilities.  A pair of dice is rolled. What is the probability of rolling double sixes?  Two cards are drawn from a deck, what is the probability of drawing a king?

The Addition Rule  The probability that either of two events will occur is the sum of their separate probabilities.  A single die is rolled. What is the probability of rolling a 5 or a 6?  A single card is drawn from a deck. What is the probability of drawing a kind or a queen?

Complete Dominance  Occurs when one form of a trait is completely dominant over the other

The Monohybrid Cross

Example 1  Brown eyes are dominant over blue eyes. Cross two parents who are heterozygous for eye color.  What is the genotypic ratio?  What is the phenotypic ratio?

Example 2  A brown-eyed woman whose father was blue eyed married a blue-eyed man whose mother was brown-eyed. What are the possible phenotypes of their children?

The Dihybrid Cross

 Brown hair is dominant over blond. Curly hair is dominant over straight. Cross a heterozygous brown curly haired woman with a man who has brown curly hair. The man’s mother had straight blond hair.

 Normal skin pigmentation is dominant over albino skin. Brown eyes are dominant over blue eyes. Cross a heterozygous, brown eyed, normal skinned female with an albino, blue-eyed man.

Incomplete Dominance  Offspring between a homozygous dominant individual and a homozygous recessive individual display a blending of the two phenotypes

R WW RW

Incomplete Dominance  Example: Snapdragons  Homozygous red flower x homozygous recessive flower= 100% pink flowers  What happens when you cross two pink flowers?

Codominance  Occurs when both dominant alleles are apparent in the phenotype of heterozygous offspring

Example 2: Blood Type

Multiple Alleles  Occurs when there are multiple alleles (more than 2) for a specific gene  Example: ABO Blood Group’

 A woman is heterozygous for type A blood. Her husband is heterozygous for type B blood. What is the probability that they will have a child with type O blood?

Pleiotrophy  Occurs when a gene has an effect on more than one phenotypic character

Epastasis  Occurs when a gene at one locus effects a gene at another locus  If gene A masks the effects of gene B, then gene A is said to be epistatic to B.

In sweet peas, two pairs of genes are at work in determining the color of the flower. C= colored flowers possible c= white flowers P= purple flowers p= white flowers What color flowers would be produced by the following genotypes: Which gene is epistatic to the other? CCPP- CCPp- CCpp- CcPP- CcPp- Ccpp- ccPP ccPp- ccpp-

Epistasis

Polygenic Inheritance  Characteristics are controlled by multiple genes  Quantitative Characters- those that vary in populations along a continuum  Skin color  Height

Recessively Inherited Disorders  Carriers- heterozygotes that carry the recessive allele, but do to having a dominant allele appear normal  Typically occur in specific groups or populations- why?

Tay-Sachs Disease  Occurs among Jewish people with ancestors from central Europe  This population is 100 times more likely to be effected  Brain cells are unable to metabolize lipids  Lipids accumulate in the brain and the child will eventually die

Cystic Fibrosis  Occurs most frequently among people with ancestors from Europe  4% of population are carriers  Defect causes a lack of chlorine ion channels, chlorine builds up outside the cells and forms a thick mucus  Lungs, pancreas, and digestive tract are most affected

Sickle-Cell Disease  Most common among those from African descent; effects 1 in 400 African Americans  1 in 10 African-Americans carries the trait  Causes a single amino acid substitution in the hemoglobin protein  Causes RBCs to become sickle shaped; cannot carry oxygen as well and can easily cause blockages  Carriers are said to have the sickle cell trait- which may be evolutionarily advantageous

Sickle Cell Disease

Dominantly Inherited Disorders  Recessive alleles for some traits are much more common than the dominant version  Achondroplasia- dwarfism  Polydactyly- extra fingers and toes

Lethal Recessive Alleles  Usually cause the death of the individual before they mature and reproduce  Huntington’s Disease  Does not cause symptoms until years of age  By this point the affected individual may have already had children  The child will then have a 50% chance of having the disorder