Inheritance

What are the two theories that emerged that explain Mendelian Laws of Inheritance?

1. Blending inheritance: gametes contain hereditary determinants that blend in the zygote. Offspring phenotypes are intermediate. 2. Particulate inheritance: hereditary determinants are distinct and remain intact at fertilization

discuss when genes recombine?

Absolute linkage is rare—genes on the same chromosome do sometimes separate. Genes may recombine during prophase I of meiosis by crossing over. Chromosomes exchange corresponding segments. The exchange involves two chromatids in the tetrad; both chromatid become recombinants

To determine if allele sex-linked

Affected mother: ALL male offspring affected • If most of males in a pedigree are affected, likely X-linked o If 50-50 male-female affected ratio, likely not sex-linked Males can only pass it allele on to their daughters X-linked, can be dominant or recessive, so watch those details)

Define allele and gene loci

An allele is a variant form of a gene. Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosome. Humans are called diploid organisms because they have two alleles at each genetic locus, with one allele inherited from each parent.

What is the difference between character and trait?

Character: observable physical feature (e.g., seed shape) Trait: form of a character (e.g., round or wrinkled seeds)

Give an example of polymorphosim

Coat color in rabbits is determined by multiple alleles of the C gene (polymorphic) Polymorphic - many alleles ○ Ex. Rabbit fur color ○ Usually there is a hierarchy of alleles ■ Dark gray allele>chinchilla>point restricted>albino

Example of independent assortment

Crossing RRYY and rryy will end up with an F1 generation of RrYy ○ Crossing two F1 peas will end up in 9:3:3:1 phenotypic ratio due to independent assortment ○ Independent assortment allows all four combination of gametes

What is epistasis?

Epistasis: phenotypic expression of one gene is influenced by another gene: ● Epistasis - multiple genes for a single phenotype ○ Example: dog fur coat color Coat color in Labrador retrievers: • For alleles B (black) and b (brown) to be expressed, allele E (pigment deposition) must be expressed. • An ee dog is yellow regardless of which B alleles are present. E is said to be epistatic to B

How were linked genes discovered?

Fruit flies have an allele for type of wings and body color ○ Expected = equal numbers of each phenotype ○ Saw a lot more the parental phenotypes than non-parental ■ The alleles (b and v) are on the same chromosome ■ Non-parental due to crossing over (new phenotypes occured due to recombination)

When can genes recombine in meoisis?

Genes can recombine during prophase/metaphase 1 of meiosis

What are sex-linked genes?

Genes located on the sex chromosomes. • X and Y chromosomes are not true homologs; many genes on X are not present on Y. • xx- female: • xy- male: Sex-linked genes were discovered in fruit flies: A gene for eye color is on the X chromosome, the Y doesn't have it. A single copy of a gene is called hemizygous.

What is an example of a rare dominant allele?

Huntington's Disease: rare dominant allele, affected individuals are heterozygotes Every person affected has an affected parent 50% probability that an individual offspring will inherit dominant allele About ½ of children of affected parent will inherit the dominant trait • Trait is present in every generation • Affected child has affected parent • Two affected parents have an unaffected child

What chromosome carries more genes? why?

In mammals, the X chromosome is larger and carries more genes than the Y, so sex-linked genes are usually on the X chromosome.

What is an example of a sexlinked disorder

Red-green color blindness: X-Linked recessive • Phenotype appears much more often in males • Daughters who are heterozygous are carriers • Mutant phenotype can skip a generation if it passes from a male to his daughter

Round, yellow seeds (RRYY) Wrinkled, green seeds (rryy) F1 generation is RrYy (all round yellow) Cross the F1 generation (double heterozygotes) in a dihybrid cross. Question: are the traits linked, or do they segregate independently?

RrYy x RrYy • If linked, gametes would be RY and ry (or just Ry and rY) - F2 would have three times more round yellow than wrinkled green (if RY and ry) • If independent, gametes could be RY, ry, Ry, or rY - F2 would have nine different genotypes; phenotypes would be in 9:3:3:1 ratio.

Sex-linked chrosomes

Sex-linked chromosome ○ Females have two X chromosomes and males have one X and one Y (males are hemizygous) ○ With only one X chromosome, males will only have one allele for the genes on the X chromosome. so males need one x chroosmoe for the trait to be expressed, while females need both x chromosomes (otherwise having one means that they are carriers) ○ In fruit flies, genes for eye color is on the X chromosome ■ Males get their X chromosome from mother, whatever allele is on this chromosome will get expressed in males

When experiments yield results that do not match expected ratios according to the law of independent assortment....

Some genes are linked. Example: Alleles: B (gray body) is dominant over b (black body) Vg (wings) is dominant over vg (small wings) Absolute linkage is rare—genes on the same chromosome do sometimes separate. Genes may recombine during prophase I of meiosis by crossing over. Chromosomes exchange corresponding segments. The exchange involves two chromatids in the tetrad; both chromatids become recombinant.

What is Mendel's second law?

The Law of Independent Assortment. Mendel's second law, stating that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes. ● Different genes (ex. gene for hair color and gene for eye color) can line up independently, resulting in different combinations of genes

What are exceptions of Mende's law of independent assortment?

When genes are not independent, they are linked ○ The genes are close enough together on the same chromosome that they are inherited together

Incomplete Dominance

When heterozygotes express an intermediate phenotype ■ Purple eggplant crossed with white will yield light purple eggplants ○ Not blended inheritance because original phenotypes of parents can be recovered. If it was blended it will not be recovered. Some alleles are neither dominant nor recessive...a heterozygote has an intermediate phenotype: incomplete dominance.

Codominance

When heterozygotes express both phenotypes ■ Flowers with red and white patches ■ Blood cell antigens (ABO)

What happens when crossing RR with rr

When the F1 self-pollinates, there are three genotypes that yield the dominant trait, but only one way to get the recessive 3:1 ratio phenotypic ratio

What is a test cross?

an unknown genotype crossed with a homozygous recessive genotype crossing an organism with a dominant phenotype with one that has a recessive phenotype progeny of a test cross can reveal whether an organism is homozyhous or heterozygous If F1 all show same characteristic, homozygous dominant If show 50:50, then heterozygous

What is Mendel's Law of Segregation?

states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization. Alleles pairs separate during gamete formation, and reunite randomly at fertilization Mendel's first law— The law of segregation: the two copies of a gene separate during meiosis; each gamete receives only one copy.

Pattern of rare recessive alleles?

• Affected people can have two parents who are not affected. • Only a small proportion of people are affected: about ¼ of children whose parents are both heterozygotes. - There has usually been a marriage of relatives Can skip a generation • A trait skips a generation and then reappears • An affected individual has two unaffected parents

What is an example of rare recessive allele disease

• Albinism: homozygous recessive individuals are affected, heterozygotes are carriers • 25% chance an individual is affected when both parents are carriers

How to determine if an allele is dominant?

• Every affected person has an affected parent. • About half of the offspring of an affected parent are also affected

Describe Mendel's experiment

• He transferred pollen from one plant to another: the parental generation, P • The seeds and offspring were the first filial generation, F1 • In some experiments the F1 plants were allowed to self-pollinate and produce a second filial generation, F2 The simplest explanation for the 3:1 ratios was that there were 4 ways to get each F2 plant and that each parent contributes equally in two ways, by contributing heritable units as discrete particles—the particulate theory. • Theory: hereditary determinants (genes) occur in pairs and segregate from one another during formation of gametes - each pea plant has two genes for each character, one inherited from each parent. Diploid/Haploid

What are some complications with how alleles interact?

• Over time genes accumulate differences and new alleles arise (mutations). • There may be more than two alleles for one character. polymorphic • Alleles don't always show simple dominant-recessive relationships. • A single allele may have several phenotypic effects or a single character could have numerous alleles Multiple genes affect one character - epistasis • Single gene have many phenotypic effects - pleiotropy ○ Mutations - new or loss of function that will lead to any of the above effects ○ Environmental interactions can also impact gene expression ○ Dominant vs recessive rule does not apply to some alleles

What is dominance inheritance?

○ One allele is completely dominant over another allele ○ Heterozygotes will show the dominant allele phenotype

Give an example of pleitropy

● Pleiotropy - one allele has multiple phenotypic phenotype ○ Sickle cell anemia - mutation in hemoglobin resulting in sickle shaped red blood cells Having both alleles for sickle cell anemia will result in multiple phenotypes such as anemia, physical weakness, pain and fever, heart failure, spleen damage, etc... ■ Why hasn't this allele been eliminated by evolution? ■ Heterozygotes have an advantage in populations exposed to malaria so the recessive allele is still passed on