Gene Linkage, Polyploidy & Punnett Squares

Gene Linkage

Chromosomes contain multiple genes

Genes that are located close to each other on the same chromosome are said to be linked

Linked genes usually travel together during gamete formation

Linkage of genes on a chromosome results in an exception to Mendel’s law of independent assortment because linked genes usually do not segregate independently

However, scientists concluded that linked genes can separate during crossing over.

Crossing over occurs more frequently between genes that are far apart

Polyploidy

Polyploidy is having one or more extra sets of all chromosomes. Example: Triploid organism 3n, Polyploidy 6n, 8n

Polyploidy is always lethal in humans

Polyploidy usually occurs most frequently in plants

Polyploidy plants usually have increased vigor and size

Punnett Squares

We use Punnett Squares to predict the possible offspring of a cross between two know genotypes

Terms to know

Homozygous both alleles for a trait are the same

Heterozygous if the alleles for a trait are different

A yellow-seed plant could be homozygous (YY) or heterozygous (Yy).

Genotype an organism’s allele pairs

YY or Yy

Phenotype the expression of the allele pair, the way an organism looks or behaves

blue eyes, brown hair, attached ear lobes, etc.

Phenotype Physical appearance

Practice Punnett Square Problems:

Cross a homozygous dominant tall pea plant with a heterozygous pea plant.  T= tall and t = short

a.  What percentage will be short? b.  What percentage will be tall? c. What are the genotypes and

phenotypes? 

In purple people eaters, one-horn is dominant (H) and no horns (h) is recessive. Draw a punnett square showing the cross of a purple people eater that is heterozygous with a purple people eater that does not have horns. Summarize the genotypes and phenotypes of the possible offspring.

SpongeBob’s aunt and uncle, SpongeWilma and SpongeWilbur, have the biggest round eyes in the family. Wilma is believed to be heterozygous for her round eye shape, while Wilbur’s family brags that they are a pure line. Complete the punnett square to show the possibilities that would result if SpongeWilma and SpongeWilbur had Children.

Give the genotype for each person. Complete the punnett square…

List the possible genotypes and phenotypes for the kids

What is the probability that the kids would have round eyes?

What is the probability that the kids would be oval eyes?