Genetics

Keystone Review

Terms You Need To Know

Gene- sequence of DNA that codes for a protein and thus determines a trait

Trait- a characteristic that an organism can pass on to its offspring through its genes.

Terms You Need To KnowAlleles- alternative forms of a gene, represented by capital and lowercase letters(You only have 2 alleles for each trait, one from your Mom and one from your Dad)

•Dominant- allele that will overpower the alternative allele (recessive). An individual will show the dominant trait as long as one of the alleles is the dominant allele.•Recessive- allele that is overshadowed by the dominant allele. The recessive trait only shows if no dominant allele is present.

Terms You Need To Know

Genotypes– The actual genes you have for a trait

TT- (homozygous dominant)

Tt- (heterozygous)

tt- (homozygous recessive)

– The letters represent actual genes inherited (one from each parent)

Phenotypes– Actual appearance (think PHoto- PHeno)– The way the trait shows itself– If T is the allele for tallness and t is the allele

for a dwarf plant:

TT- tall plant

Tt- tall plant

tt- dwarf plant

Terms You Need To Know

An example

There is an allele for detached and an allele for attached. The allele for detached is dominant. The allele for attached is recessive.If your genotype is homozygous dominant (EE) your phenotype is detached earlobes.

If your genotype is heterozygous (Ee) your phenotype is detached earlobes.

If your genotype is homozygous recessive (ee) your phenotype is attached earlobes.

There is a gene that determines the trait of having detached earlobes or attached earlobes.

Solving Genetics Word Problems

• On the keystone test you will be expected to answer multiple choice questions and written response questions about inheritance patterns.

• Punnett squares can be used to show the possible outcomes for a trait according to the traits of the parents. They also show the probability of the outcome

Ee x Ee

• Squares represent possible offspring

• Each offspring gets one gene from “dad” (from the top) and one gene from “Mom” (from the side)

• What % of the offspring will have detached earlobes?

• What % will have attached earlobes?

• What % will be heterozygous for the trait?

EE

Ee

Ee ee

E

e

E

e

Mendelian Genetics

• Medelian Genetics describe situations in which the basic rules are followed as seen thus far.

• The first part of your worksheet are regular dominance (or Mendelian Genetics) problems

• There are other inheritance patterns however, that go beyond this.

Other Inheritance Patterns

Going Beyond Simple Dominance:1. Incomplete Dominance2. Codominance3. Multiple Alleles4. X- linked (or sex linked) Traits5. Polygenic Traits

Incomplete Dominance• In some cases, neither allele

truly dominates over the other. No allele is really recessive.

• The heterozygous genotype shows a MIX of the two traits.

• Examples- Four O’Clocks, Snapdragons– R- gene for red flowers, W- gene

for white flowers:– RR- red, WW- white, RW- pink

Codominance

• In some cases, both alleles are dominant. No allele is really recessive.

• The heterozygous genotype shows BOTH of the two traits.

• Example- Chicken feathers– B- gene for black feathers, W- gene for white

feathers:– BB- black, WW- white, BW- “erminette” Black and White!

Multiple Alleles

• Many genes have more than just two alleles for a trait

• Example: Alleles for rabbit fur– C-full color, dominates over Cch, Ch, c– Cch – chinchilla, dominates over Ch and c– Ch- himalayan, dominates over c– c- albino, recessive to all

Practice Cross C chCh x cc

• What are the phenotypes of the parents?

• Fill in the Punnett Square.• What is the probability

that an offspring will be albino?

• What is the probability that an offspring will be himalayan?

Human Blood Types

Multiple alleles and Codominance!

•Multiple alleles- IA, IB, i

•Codominance- A and B are codominant but both dominate over O

Sex-linked Traits

• Often called X- linked traits

• Trait can be dominant or recessive

• Probability of inheritance is altered because the trait is on the X chromosome

• Females- XX Males- XY

Examples of X-linked traits

• Red-green colorblindness

• Hemophilia• Duchenne

Muscular

Dystrophy

Polygenic Traits

• Many traits result from the interaction of several genes.

• Polygenic traits can produce a large range of phenotypes

• Examples: human skin color (at least 4 genes), human eye color, human height

Gradation of Human Skin Color

Meiosis

The alleles that Mendel was using represent traits which are actually coded in genes, just small sections along the DNA. The DNA, remember, is coiled and condensed into chromosomes. NOW:

How do parents actually pass on traits to children?

• DIPLOID– Cells with a double set of

chromosomes– “2n”– Humans – 23 sets/pairs,

2n=46

• HAPLOID– Single set– “n”– Humans – one chromo from

each set, n=23

meiosis

• Similar to mitosis BUT…– 2 phases of chromosome separation– G1 S G2 Meiosis I Meiosis II– Produces 4 gametes with half the normal

chromosome number– 1 diploid somatic (normal cell) 4 haploid cells– Distributes chromosomes “randomly” into

gametes– One chromosome from each homologous pair

Tetrads line up at equator

Homologous chromosomes separate, NOT chromatids

Crossing over

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase II

The chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Spermatogenesis vs. Oogenesis

Russel p. 74

In summary:

Keystone-like Questions:

In a flowering plant species, red flower color is dominant over white flower color. What is the genotype of any red-flowering plant resulting from this species?

A. red and white alleles present on one chromosomeB. red and white alleles present on two

chromosomesC. a red allele present on both homologous

chromosomesD. a red allele present on at least one of two

homologous chromosomes

Mitosis and meiosis are processes by which animal and plant cells divide. Which statement best describes a difference between mitosis and meiosis?

A. Meiosis is a multi-step process.

B. Mitosis occurs only in eukaryotic cells.

C. Meiosis is used in the repair of an organism.

D. Mitosis produces genetically identical daughter cells.

Blood type is inherited through multiple alleles, including IA, IB, and i. A child has type A blood. If the father has type AB blood, what are all the possible phenotypes of the mother?

A. phenotypes O or A

B. phenotypes A or AB

C. phenotypes A, B, AB

D. phenotypes O, A, B, AB

Sample Written Response:

Patau syndrome can be a lethal genetic disorder in mammals, resulting from chromosomes failing to separate during meiosis.•Part A: Identify the step during the process of meiosis when chromosomes would most likely fail to separate.•Part B: Describe how chromosome separation in meiosis is different from chromosome separation in mitosis.•Part C: Compare the effects of a disorder caused by chromosomes failing to separate during meiosis, such as Patau syndrome, to the effects of chromosomes failing to separate during mitosis.