Genetics of Maize

Genotype

Genetic makeup of a trait

Phenotype

Physical manifestation of a trait

Loci

Designated locations on a chromosome

Genes

Composed of DNA

What does DNA code for?

Proteins

What do proteins do?

Carry out a particular function that result in an organism’s phenotype

What happens if DNA that codes for a gene varies?

Produces a protein that may be more or less effective at carrying out its function

What determines an organism’s phenotype?

How effective a protein is

What chromosomes do diploid organisms have?

Homologous chromosomes

Mendel’s Law of Segregation

One chromosome came from the organism’s father and the other from its mother, each pair has same type of alleles on it usually on the same locus

Mendel’s Law of Independent Assortment

Based on the idea that most organisms have more than one pair of homologous chromosomes

During Meiosis the separation of one pair of homologous chromosomes does not affect the separation of another pair = alleles on diff chromosomes do not travel together, they are independent.

Diploid have a pair of what

Chromosomes

Where does Mendel’s Law of Segregation take place?

Anaphase 1 of meiosis

What happens during fertilization

One gamete from each parent joins, creating a new combo of homologous chromosomes

Mendel’s Law of Independent Assortment

During meiosis the separation of one pair of homologous chromosomes does not affect the separation of another pair = alleles on different chromosomes do not travel together, they are said to be independent

Model organism traits

• Survive well in lab

• Provide sufficient numbers for statistical analysis

• Have a short life cycle so multiple generations can be observed

Punnett Square

Way of keeping track of alleles in parents and potential offspring

1:1 relationship

There is a 50/50 chance offspring will be one or the other phenotype

Dihybrid Cross

Outcome of two different (usually independent genes) are predicted; all possible combos in the punnett square because of independent assortment

Why are ratios of given offspring in Punnett Square expected and not observed

Assortment of homologous chromosomes during meiosis is by chance like fertilization by particular gametes

Chi Square

Method that allows us to determine whether what is expected actually occurs

Null hypothesis

A statistical hypothesis to be tested and either accepted or rejected in favor of the alternative; gives us gametes and ratios shown in a Punnett Square

What happens if observed is significantly different than expected?

Variations not due to chance and hypothesis must be rejected

What is the embryo

2n

What does the embryo feed off of during early growth?

Endosperm

What does endosperm develop off of

3n double fertilization

Aleurone

Outer layer of the endosperm

Pericarp

Surrounds all of the above and is tissue from the parent (2n)

What do phenotypic traits of corn seed involve?

Aleurone color and endosperm texture and color

What genes control the production of anthocyanin pigment (purple or red) in aleurone

A,C, and R

Do ACR genes segregate independently?

Yes because they are located on separate chromosomes

What is needed to get anthocyanin production in the ACR loci?

At least one dominant allele

What does the color obtained depend on?

Other independent loci, Pr

What does Pr (aleurone gene) in dominant state result in?

Purple aleurone

What does the recessive state of aleurone gene result in (prpr)

Red pigmentation

What does homozygous recessive condition in these genes do?

Prevent the production of anthocyanin and thus results in a colorless aleurone = endosperm is expressed

What other gene inhibits color production in aleurone regardless of the genotypes of A,C or R loci

CI (allele of C locus)

What causes normal corn endosperm color?

Yellow when gene Y causes the production of carotenoid pigments (yellow) in the endosperm

What does recessive condition lead to (yy)

Lack of carotenoids makes endosperm appear white

What is the observed color of various corn kettles a result of?

Combo of endosperm and aleurone layer colors

What does dark aleurone layer do?

Masks endoderm color

What does colorless aleurone do?

Permits expression of endosperm color, yellow or white

What can corn alleles control?

Smooth or wrinkled appearance of the kernel

What is normal appearance of corn

Starchy- amylose type of starch

What is the gene that separates sweet corn and field corn

Sugary locus

Su homozygous recessive (susu)

Produces endosperm with very high sugar content and wrinkled and transluscent appearance when kernels are dried

Dominant allele of susu

Starchy endosperm with dried kernels that are opaque and smooth in appearance

What do gene actions involved with corn kettle depend on?

The action of single genes two genes or more

Recessive epistasis

Involves the case where one gene pair masks the expression of other genes = homozygous recessive at one locus masks expression of all alleles at second locus (recessive epistasis)

Dominant epistasis

Dominant allele at one locus can mask the expression of second independent loci

What gene in corn demonstrates epistasis?

Dominant CI allele prevents production of aleurone color regardless of alleles at the A R or Pr loci