Chapter 12-1 Chromosomes and Inheritance patterns

Objectives

• Explain the role of sex chromosomes in sex determination

• Describe how sex linkage affects the inheritance of traits

• Explain the effect of crossing over on the inheritance of genes in linkage groups

• Summarize the procedure involved in constructing a chromosome map

• Distinguish between chromosome mutations and gene mutations

• How does the inheritance of sex chromosomes result in approximately equal numbers of males and females among the offspring of fruit flies.

• Shown in a punnet square

• Offer an explanation for why morgan did not find white-eyed female drosophilia in the F2 generation when he crossed white eyed males with red eyed females.

• The eye color gene is located on the X chromosome and any female offspring would be heterozygotes

• How does crossing over show that genes are found on chromosomes

• Because of the expression of those genes in the offspring

• How can crossing over between two alleles be used to map their locations on chromosomes

• To make the map scientists mate organisms together and watch how often the genes get split up

• Each percent of the time they get split up is one map unit.

• By comparing many genes scientists can figure out the order they are in

• What are point mutations

• Changes in one nucleotide in DNA

Sex Determination

• T.H. Morgan bred the fruit fly Drosophilia Melanogaster to study genetics– Fruit Flys are good to study– They have lots of offspring– They live a short time– They are easy to take care of– They only have four pairs of chromosomes

• In fruit flies XX are females, and XY are males– Just like humans

• Like other chromosomes the X and Y pairs segregate during Meiosis

Sex Linkage

• Since the X is bigger than the Y, Morgan hypothesized that there would be more genes on the X

• Genes on the X chromosome are called X-linked Genes

• Genes are the Y chromosome are called Y- linked genes

• This is sex linkage

Inheritance of sex linked genes

• For example in fruit flies the gene for eye color is on the X-chromosome

• Red is dominant and white is recessive

• In Sex-linked genes the recessive trait shows up more often in males than in females

• Why?

Y-linked• SRY gene• SRY (Sex-determining Region

Y) is a sex-determining gene on the Y chromosome in humans and other primates.

• One of the most controversial uses of this discovery was as a means for gender verification at the Olympic Games,

• Athletes with a SRY gene were not permitted to participate as females, although all athletes in whom this was "detected" at the 1996 Summer Olympics were ruled false positives and were not disqualified. In the late 1990s, a number of relevant professional societies in United States called for elimination of gender verification,

Linkage groups

• Each chromosome carries many genes

• All the genes on a chromosome form linkage groups

• Since they are linked on the same chromosome these genes tend to get inherited together

Fruit Flies again

• Morgan crossed Gray-long winged flies with black-short winged flies

• Using two traits you’d think it would look like the big dihybrid punnet squares

• The results actually looked like a little four square punnet square because color and wing length are linked on the same chromosome

Crossing over happened

• Sometimes Morgan got Black long winged and Grey Short winged flies

• If color and wing length are on the same chromosome how did they get mixed?

• During meiosis some parts of chromosomes switch off

Chromosome Mapping

• If two genes are far apart on a chromosome and crossing over happens they are more likely to get split up

• If two genes are close together on a chromosome they are less likely to get split up

• A chromosome map is a diagram that shows the order of genes on a chromosome.

• To make the map scientists mate organisms together and watch how often genes of interest separate.

• 1 Map unit = 1 % recombination frequency. In other words, divide the number of recombinant offspring by total offspring, then convert to percent.

• By comparing many genes scientists can figure out the relative positions of the genes on the chromosome.

Mutation

Mutations can involve an entire chromosome or a single nucleotide (the ATGC letters)

Germ cell mutations occur in an organism’s gametes. These are the mutations that can be passed on.

Somatic mutations happen in your body cells and are not passed on.

Chromosome mutations• A deletion is a loss of a piece of a chromosome due to breakage

•Inversion: a segment breaks off and reattaches in the opposite direction

•Translocation: a segment breaks off and reattaches onto another chromosome

•Nondisjunction: two chromosomes fail to separate during meiosis

Gene mutations

• Point mutations: when a single nucleotide mutations

• Substitution mutations happen when one nucleotide is replaced with another

• This leads to a different codon dif. Amino acid dif shaped protein.– Sickle Cell is an example

Frame-shift mutation

• The insertion or deletion of a nucleotide switches the order in which the codon’s are read and can be much more serious than a substitution.

• How does the inheritance of sex chromosomes result in approximately equal numbers of males and females among the offspring of fruit flies.

• Shown in a punnet square

• Offer an explanation for why morgan did not find white-eyed female drosophilia in the F2 generation when he crossed white eyed males with red eyed females.

• The eye color gene is located on the X chromosome and any female offspring would be heterozygotes

• How does crossing over show that genes are found on chromosomes

• Because of the expression of those genes in the offspring

• How can crossing over between two alleles be used to map their locations on chromosomes

• To make the map scientists mate organisms together and watch how often the genes get split up

• Each percent of the time they get split up is one map unit.

• By comparing many genes scientists can figure out the order they are in

• What are point mutations

• Changes in one nucleotide in DNA