00:01
Male beetles can either have horns or lack horns.
00:04
The females never have horns.
00:07
And this is a single gene and the horned allele is dominant.
00:11
So we have capital h for horns, lowercase h for hornlose.
00:19
Okay, so this is what we call a sex -limited trait.
00:24
A sex limited trait isn't on a sex chromosome, but its expression is still affected by the sex of the individual.
00:31
In this case, even if a female has one or two copies of this dominant allele, she does not develop horns.
00:39
So, we're crossing a hornless male with a female, and the female is always hornless.
00:45
And what are the parental genotypes here? well, the male, we know his genotype, a hornless male is always homozygous recessive.
00:56
But the female is a complete mystery.
00:59
We don't know if she has this allele or not.
01:02
We've got no idea.
01:03
So let's look at the offspring and work it out.
01:06
So their first offspring has horns.
01:09
The second has horns.
01:11
The third, lax horns.
01:14
Out of a hundred offspring, how many males will have horns? so we are trying to work out the females genotype based on their offspring.
01:24
Remember, each offspring gets one allele from each parent.
01:27
So all of the three beetles we're looking at inherited a...
01:33
Recessive allele from a male.
01:37
The others all inherited one of these question marks.
01:40
These could be the same, they could be different.
01:42
They're going to be different because we're getting different outcomes here.
01:45
This one has horns, so it must have a copy of the dominant.
01:52
This one also has horns, so it also has a copy of the dominant.
01:59
This one is male, but lacks horns, which means it must have inherited a recessive allel...