Test 3 #2

Leaf shape is a Mendelian trait in certain species of tomato. How could you determine which leaf shape, rounded or pointed, is dominant and which is recessive?

- Cross a true-breeding round leaf tomato plant with a true-breeding pointed leaf tomato plant. All of their offspring will have the dominant trait for leaf shape. ***A true-breeding tomato plant is homozygous for the leaf shape gene. Therefore, the offspring of a cross between a true-breeding round leaf tomato plant and a true-breeding pointed leaf tomato plant will all be heterozygous for the leaf shape gene. For Mendelian traits, the dominant trait is the trait observed when two different alleles are present. Dominant traits are NOT necessarily more beneficial for an organism compared to recessive traits and are NOT necessarily more prevalent in the population.

For Mendelian traits, an individual that has the genotype EeGGcc will have the same phenotype as an individual with the genotype _________.

- EEGgcc *** Individuals with one or two alleles that correspond to a dominant trait have the same phenotypes, so EE and Ee have the same phenotype, as do GG and Gg. The recessive trait is ONLY observed when two alleles that correspond to the recessive trait are present (i.e. to have the same phenotype as an individual that is cc, the other individual MUST also be cc).

In a species of fly, the genes for body color and wing shape are on different chromosomes. A fly has the black body allele and the gray body allele for the body color gene, and the pointy wing allele and the rounded wing allele for the wing shape gene. Some of the gametes produced by the fly have the black body allele and the rounded wing allele, while other gametes have the black body allele and the pointy wing allele. What mechanism of genetic variation explains this?

- Independent Orientation ** Independent orientation mixes the alleles an individual inherited from each parent that are on different chromosomes when gametes are produced by meiosis. The image below illustrates this. Because the genes for body color and wing shape are on different chromosomes, crossing over does NOT apply.

An organism has the genotype AaBb. What proportion of its gametes, if any, would be Bb?

- None *** Gametes are haploid, meaning that they only have one of each chromosome (NO homologous pairs), and therefore can only have ONE allele for each gene. Alleles of the same gene separate during meiosis when homologous chromosomes are separated (specifically in meiosis I). The possible gametes for an organism that is AaBb are AB, Ab, aB, and ab. Each is equally likely (probability is 1/4 for each).

Flower color and petal spots are both Mendelian traits in a species of flower, and the genes that determine the characters are on different chromosomes. Flowers can be either orange or yellow, and petals can be either spotted or plain (no spots). When a plant that is heterozygous for both genes is self-fertilized, it produces the following offspring: 54% plants with orange flowers and spots, 21% plants with yellow flowers and spots, 20% plants with plain orange flowers (no spots), and 5% plants with plain yellow flowers (no spots). Based on the information provided, the dominant trait for petal spots is _____________, and the dominant trait for petal color is _____________. HINT - Focus on each character (flower color and flower spots) separately.

- spotted; orange **** Because flower color and flower spotting are Mendelian traits, they're examples of complete dominance (determined by one gene; heterozygotes show trait that corresponds to the dominant allele). To determine the dominant trait for each character, look at the total percentage of offspring that have each trait.

What event in meiosis explains why gametes are haploid?

Homologous chromosomes are separated in meiosis I.

What process during meiosis explains why the combination of alleles carried on the same chromosome can be different in the gametes produced by meiosis than in the parent cell that divides?

Homologous chromosomes can exchange pieces of DNA (crossing over)

The F1 offspring are all heterozygous for both genes (AaBb). If ALL the gametes produced in an F1 offspring are AB or ab (there are no Ab or aB gametes), what must be true of the two genes?

The two genes are on the same chromosome and no crossing over occurred between the genes during meiosis.

In zebrafish, long fins are completely dominant to short fins. A breeder wants to set up a breeding program beginning with homozygous long-finned fish. She has a group of fish with long fins, but does not know which ones are homozygous for the long fin allele, and which ones are heterozygous. The breeder crosses each of the long-finned fish to a short-finned fish, and observes the fin length of the offspring. To identify the long-finned parent fish that are homozygous, she should pick ones where....

all the offspring have long fins.

You are assessing three specific genes. Determine the number of possible gamete genotypes (for those genes) for an individual that is heterozygous for all THREE genes.

eight

In a diploid species of alien (MendAliens) that reproduces sexually, eye color and skin color are Mendelian traits. Black eyes are completely dominant to orange eyes, and green skin is completely dominant to white skin. Sam, a MendAlien with black eyes and green skin, has a parent with orange eyes and white skin. Carole is a MendAlien with orange eyes and white skin. If Sam and Carole were to mate, what are the possible offspring phenotypes and their probabilities? HINT - Figure out Sam and Carol's genotypes and use them to determine the possible gamete genotypes for each alien (use whatever letters you want for the alleles). Then set up a Punnett square. The information about Sam's parent is important!

- 1/4 black eyes, green skin ; 1/4 black eyes, white skin ; 1/4 orange eyes, green skin ; 1/4 orange eyes, white skin ***** Sam's genotype MUST be BbGg. Sam has a parent that shows both recessive traits (orange eyes and white skin). Therefore, one of Sam's parents was homozygous recessive (two alleles that correspond to the recessive trait) for both the eye color gene and the skin color gene. This means that Sam inherited the orange eye allele and white skin allele from that parent. Since Sam shows both dominant traits (black eyes and green skin), he must have inherited the black eye color and green skin color alleles from his other parent. Carole's genotype MUST be bbgg because she shows the recessive trait for both skin color and eye color. Sam can produce BG, Bg, bG, or bg gametes. Carole's gametes can ONLY be bg. You can set up a Punnett square to see that there are four possible offspring genotypes if Sam and Carole mate, and each genotype is equally likely to occur. Each of the genotypes also relates to a unique phenotype, so there are four different offspring phenotypes that are equally likely to occur.

You are studying two Mendelian traits in flies, bristle shape and antenna length. Bristles are either straight or curly and antennas are either short or long. You cross a fly that is homozygous for both straight bristles (A) and short antennas (B) with a fly that is homozygous for both curly bristles (a) and long antennas (b) to obtain F1 offspring. What do you know about the genotypes of the F1 offspring?

- All off the offspring will be heterozygous for both genes **** Both of the parent flies are homozygous for both traits. This means the parent that's homozygous for straight bristles and short antennas (AABB) can only produce gametes with a straight bristle allele and a short antenna allele. And the parent that's homozygous for curly bristles and long antennas (aabb ) can only produce gametes that have a curly bristle allele and a short antenna allele. Therefore, the offspring will all be heterozygous for both genes.