In cucumbers, heart-shaped leaves (h l) are recessive to...

In cucumbers, heart-shaped leaves $(h l)$ are recessive to normal leaves $(H I)$ and having numerous fruit spines $(n s)$ is recessive to having few fruit spines $(N s) .$ The genes for leaf shape and for number of spines are located on the same chromosome; findings from mapping experiments indicate that they are 32.6 m.u. apart. A cucumber plant having heart-shaped leaves and numerous spines is crossed with a plant that is homozygous for normal leaves and few spines. The $F_{1}$ are crossed with plants that have heart-shaped leaves and numerous spines. What phenotypes and phenotypic proportions are expected in the progeny of this cross?

Key Concepts

Test Crosses

A test cross is a genetic cross performed to determine the genotype of an individual displaying a dominant phenotype. By crossing the test subject with an individual homozygous recessive for the trait, the resulting offspring’s phenotypes can reveal whether the subject is homozygous dominant or heterozygous based on the presence of any recessive phenotypes.

Genetic Linkage

Genetic linkage refers to the phenomenon where alleles at different loci are inherited together more frequently than would be expected by chance because the genes are located physically close to each other on the same chromosome. This reduces the independent assortment of these genes during meiosis, meaning that the alleles tend to be co-inherited unless a crossover event separates them.

Recombination Frequency

Recombination frequency is a measure of how often crossovers occur between two linked loci during meiosis and is expressed as a percentage or in map units. The frequency of recombination provides an estimate of the physical distance between the genes—the greater the map unit value, the further apart the loci, and the higher the likelihood of a crossover occurring between them.

Dominance Relationships

Dominance relationships describe how different alleles interact to produce a phenotype, where one allele may mask the expression of another in a heterozygous organism. In a typical scenario involving dominant and recessive alleles, the phenotype of the dominant allele is observed in the heterozygote, while the recessive trait only manifests when the organism has two copies of the recessive allele.

Transcript

00:01 Hey there.

00:01 Today we're doing genetics problems on heart -shaped leaves, normal leaves, and fruit spines in cucumber plants.

00:18 So we're going to go with that.

00:23 Okay, so we are given some information in this question, and we need to write down some things first.

00:29 So first off, i want you to write down these things.

00:32 We know that lowercase equals recessive, upper case equals dominant.

00:37 That's basic genetics whenever in doubt.

00:40 Just remember that.

00:40 Sometimes it might be different for people, but for our work today, that's what it is.

00:48 Okay, so from this we see heart -shaped leaves, normal leaves, numerous fruits, finds.

00:51 So i wrote down what i'm going to be notating the mess.

00:55 Then i kind of like paraphrase what they wrote, a cucumber plant having heart -shaped leaves and numerous finds, right? so then i write down the genotype.

01:05 Cross.

01:05 So this is a phenotype.

01:08 This is the genotype.

01:09 Cross, that's what the x means, a plant that is homozygous for normal leaves and fused spines.

01:17 H .i.

01:19 You see the dominance here? anyway, we cross those together, homozygous recessive homestigous dominant.

01:27 We are getting, do, do, do, do, do.

01:31 We're trying to figure out what are the percentages of, what are the phenotypic proportions and genotypic proportions? i mean the phenotypes and the phenotypic proportions.

01:50 So let's work with that.

01:52 So we are going to have, we are given some information already.

01:59 So we know this is like just something that you need to know in general.

02:10 Sorry, i'm trying to switch it to.

02:17 Here we go.

02:19 So this is just something you need to know in general.

02:24 Distance between the genes, map units, equals the recombination frequency...