GENETICS

Genetics: The study of how traits encoded in our DNA are passed on. Used to predict the possible outcomes of a genetic cross –Depends on probability –We might expect 2/4 offspring to look a certain way, and end up with 0/4 looking that way!

History of Genetics as a science: Gregor Mendel is the “Father of Genetics” - Austrian monk who studied pea plants to figure out how parents pass traits to their offspring - Started with parents that were True breeding - means that when they self fertilize, their offspring all look like them.

Parents: (both true breeding) white x purple Expect??? What he got: So… he crossed two of them…. Expect??? What he got:

These crosses showed that there were “factors” being passed from parent to offspring even if it wasn’t being “used” Now we call these factors GENES

Genes – control a heritable feature; Example: Hair color, seed shape, height; Allele – controls the variation of a feature. Example: brown, blonde, black hair

Characteristic/Gene? Trait/Allele?

CHARACTERS AND VARIANTS OF EACH

TRAITS ?

?

RARE DOMINANT PHENOTYPE

A chromosome = folded up string of many genes

What are alleles? Variations of a gene that occupy the same locus on homologous chromosomes Locus = position on a chromosome.

P p T t LONG SHORT GENE = FLOWER COLORGENE = STEM LENGTH

Terms Diploid (2n) HAPLOID (n) Egg Sperm Parent Meiosis Testes Gamete Zygote Progeny Offspring Fertilization Ovary

Mendel’s laws of genetics 1.Law of segregation: only one allele for each gene is passed from a parent to the offspring. Why? Has to do with separation of homologous chromosomes during meiosis.

Segregation of Alleles

Tongue Rolling

2. Law of independent assortment: Alleles for different genes are passed to offspring independently of each other. The result is that new combinations of genes present in neither parent is possible.

3. Law of complete dominance – some alleles over power others. So even if both alleles are present, we only “see” the dominant one. - the “hidden” allele is called recessive This only applies to SOME genes, not all.

Remember Mendel’s pea plants? - Purple was crossed with white and we got ALL purple. So which allele is dominant?

Genotype: the alleles that an organism has. - alleles are abbreviated using the first letter of the dominant trait. (with some exceptions that we will get to) - a capital letter represents the dominant ex: P for purple flower allele - a lower case represents the recessive. ex: p for white flower allele

All diploid organisms have two alleles for each trait: - you can have two of the same alleles Ex: PP or pp - such an individual is described as Pure or Homozygous. OR

All diploid organisms have two alleles for each trait: - you can have two different alleles Ex: Pp - such an individual is described as hybrid or heterozygous

Phenotype: physical appearance Examples: brown hair, widows peak - the trait that “wins” in the case of complete dominance; - depends on the combination of alleles

P generation: “parents;” First generation in the cross F generations: results of the cross; - F1 – 1 st generation; offspring of P generation - F2 – 2 nd generation; offspring of F1 generation Terminology for Genetic Crosses

Monohybrid cross: cross that focuses on the alleles of a single trait; How do we show the possibilities? - punnett square

PUNNETT SQUARE Allele in Egg 1 Zygote formed if sperm 1 fertilizes egg 1 Allele in Egg 2 Allele in sperm 1 Allele in sperm 2 Zygote formed if sperm 2 fertilizes egg 1 Zygote formed if sperm 1 fertilizes egg 2 Zygote formed if sperm 2 fertilizes egg 2

In pea plants tallness is dominant to short or dwarf. Cross a pure tall male to a pure dwarf female pea plant. Show both ratios phenotype & genotype for the offspring. Now cross two of the F1.

Take it step by step until you “get it” Step 1: what are the parent’s genotypes? –Mom? –Dad? tt TT

Step 2: Set up Punnett Square tt T T Tt Tt Tt Tt

Step 3: ANSWER THE QUESTION tt T T Tt Tt Tt Tt Offspring genotypes: Offspring phenotypes:

Step 4: Part II Tt T t TT Tt tt Tt Offspring genotypes: Offspring phenotypes:

Inheritance Patterns: Every gene demonstrates a distinct phenotype when both alleles are combined (the heterozygote) Complete dominance is one - when both alleles are present, only the dominant trait is seen.

Inheritance Patterns: Incomplete dominance - when both alleles are present, the two traits blend together and create an intermediate trait

INCOMPLETE DOMINANCE

Inheritance Patterns: Co-dominance - when both alleles are present, both traits are visible Different notation: Use first letter of the feature with a superscript for the trait. Example: C W or C B for white coat or black coat;

Inheritance Patterns: Co-dominance - when both alleles are present, both traits are visible

Inheritance Patterns: Each gene has a specific inheritance pattern. - you will either be told or be given a hint; look at the heterozygote!

Women have two X’s but men only have one. How do we deal with the genes on the X chromosome?

Sex-linked trait Alleles for the trait are located on the X chromosome in humans. - works the same in women as all the other traits. BUT – - men only inherit one such allele.

Sex-linked trait For females: have to figure out phenotype based on inheritance pattern. For Males: phenotype is that of whatever allele they inherit.

Example: color blindness Seeing color (X C ) is dominant to being color blind (X c ) Identify the sex and trait of the following: X C YX C X c X C X C X c X c X c Y

Example: Color Blindness Set up a punnett square crossing a heterozygous normal female with a normal male: - what is mom’s genotype? - what is dad’s genotype? - what gametes can each give? - what are the offspring’s geno’s?

XCXC Cross Number 1: XcXc XCXC Y XCXC XCXC XCXC XcXc XcXc Y XCXC Y What % chance of having color blind daughter? Son?

SEX-LINKED TRAITS COLOR BLINDNESS AFFLICTS 8% MALES AND 0.04% FEMALES.

If we are dominant, how can we figure out our genotype? What are the possibilities?

Test cross: a cross that determines genotype of dominant parent - Cross unknown dominant parent (possibilities BB or Bb) with a recessive parent then analyze the offspring.

B? b b Bb ?b ?b Bb If some of the offspring have the recessive trait, then the unknown parent has to be heterozygous

B? b b Bb ?b ?b Bb If all offspring are dominant, unknown parent HAS to be homozygous

Multiple alleles: Some genes have more than two variations that exist, although we still only inherit 2 Example: Human blood types Three alleles: I A I B i GenotypePhenotype IAIAIAIA A IAiIAiA IBIBIBIB B IBiIBiB IAIBIAIB AB ii0

Polygenic – Multiple genes each with 2 alleles Creates additive/ quantitative effect SKIN PIGMENTATION

Dihybrid cross: A cross that focuses on possibilities of inheriting two traits - two genes, 4 alleles Black fur is dominant to brown fur Short fur is dominant to long fur What is the genotype of a guinea pig that is heterozygous for both black and short fur?

Dihybrid cross: Parent phenotypes: BbSs x BbSs Figure out the possible gametes: Then set up punnett square

Dihybrid cross: BSBsbSbs BS Bs bS bs

Linked Genes: genes that are on the same chromosome. Does the law of independent assortment apply? Can they be separated? Will they always separate?

What does this mean? It means that you can pass on an allele that you got from your mom and an allele you got from your dad ON THE SAME CHROMOSOME However, it is more likely that two alleles that start on the same chromosome will get passed on together.