Monohybrid crosses or pedigree problem (1 Viewer)

[Shrutzzz]

hey
I am really confused with this monohybrid cross question , I mean which is the dominant breed out of the two? Well here's the question:

Lincoln red cattle are entirely red. Hereford cattle have a white face. The white face is due to a dominant allele for which the hereford breed is homozygous. A farmer has a herd of Lincoln red cows and Lary , a Lincoln red bull. Larry was allowed to mate with the cows but died before he had fertilised the whole herd. The farmer borrowed a neighbour's Hereford bull called herbert to finish larry's job.
1. he kept one of herbert's male offspring to replace larry. When this bull was mated to the lincoln red cows what would be the phenotypes of the calves and in what proportions.

shrutzzz

 

[whatdafork]

Hmmm, I'm going to assume that when you refer to the Lincoln red cattle as entirely red you also mean that it has a red face. Since the white face in the hereford cattle is due to the dominant allele which expresses white in the face (let's call it W for now) and it is homozygous, then its genotype shoud be WW.

The Lincoln cattle would have the phenotype of the red face, so I assume that the allele for red faces is recessive (let's call it w). Since the allele for white faces is dominant the red cattle cannot be heterozygous (i.e Ww) as all heterzgyous cattle are all white faced. So all the red cattle are also homozygous with both recessive alleles (ww) expressed in their genotype.

So the domiant breed should be the Hereford cattle.

Now Herbert's offspring will all have come about through a cross with a homozygous white cattle (WW) and a homozygous red cattle (ww). All the offspring that have resulted all all heterzygous with the genetype (Ww). So if you do a further cross with the offspring and the red cattle, then these results should be possible.

Ww crossed with ww
Offspring: Ww, Ww, ww, ww

The result is two heterzygous White faced cattle (Ww) and 2 homozygous Red cattle (ww).

I haven't been taught this by a teacher yet, as I just went over some of this stuff these holidays so some parts or even all of it might be wrong.

 

[kami]

hey
I am really confused with this monohybrid cross question , I mean which is the dominant breed out of the two? Well here's the question:

Lincoln red cattle are entirely red. Hereford cattle have a white face. The white face is due to a dominant allele for which the hereford breed is homozygous. A farmer has a herd of Lincoln red cows and Lary , a Lincoln red bull. Larry was allowed to mate with the cows but died before he had fertilised the whole herd. The farmer borrowed a neighbour's Hereford bull called herbert to finish larry's job.
1. he kept one of herbert's male offspring to replace larry. When this bull was mated to the lincoln red cows what would be the phenotypes of the calves and in what proportions.

shrutzzz

I think there is some information missing...are the Lincoln cattle heterozygous or homozygous? And are they dominant or reccesive?

However assuming that the Lincoln cattle are reccessive and homozygous, we have:
STEP 1 - Hereford bull(BB) with Lincoln cows(bb)
BB x bb which gives all offspring the genes(genotype) Bb and the appearance(phenotype) of a Hereford.

STEP 2 - Mixed(Bb) with Lincoln cows(bb)
Bb x bb which gives offspring with the genotypes Bb and bb in an equal ratio (ie 1:1 or 50/50), those offspring with Bb genes will have the phenotype of the Hereford and those offspring with bb will have the phenotype and genotype of a homozygous Lincoln.

It will all become much clearer if you are new to this if you do this in punnet squares.

 

[Shrutzzz]

hey thanks a lot
i get it now but there wasn't any information missing it was just worded that way i guess.
shrutzzz

 

[OzV]

umm - this question is a little strange, and none of you are close really (by no fault of your own - I have taught biology for nearly 13 years and also taught agriculture and have researched cattle genetics and have my own breeding program in place so I like to think I know a bit about it ) c ept for your final genotype ratios - it'syour phenotypes which are wrong (and that's what he q asked for...)

Ok...

1. An all red cow is homozygous for the non-white faced version of the white faced gene. The lincoln cow is therefore homozygous recessive (actual letter allocation for this gene is w - so they are ww) for the white face gene and is showing no expression of white on the face at all.
2. the hereford cow (it is not a sex linked gene so it is irrelevant whether it is a cow or a bull) is homozygous dominant for the white faced gene (WW)- BUT the white faced gene not only causes white on the face but also white on the belly regions from front to back and down the hocks.
3. The white faced gene is INCOMPLETELY DOMINANT. This means you will get an intermediate expression of both genes at this locus. The resulting cow will be mostly red with a partly white face and red belly.
4. Herbert (WW) was mated with a red cow (ww) making what's called in the cattle industry a red baldy (a white faced red) - genotypically it would be Ww. If you were to mate this to one of the lincoln cows (ww) you would get an expected ratio of 50:50 of white faced reds and reds and NO hereford patterns at all.

 

[kami]

I'm not trying to be deliberately disagreeable since I know absolutely nothing about agriculture - but how did you come to that answer without using extra information?

Its just that while I remember being taught the most simple of genetics in the HSC, I don't recall the specific traits of Hereford and Lincolns being a part of the syllabus in Blueprint of Life. So wouldn't we go according to the information given rather than what we know may happen according to extracurricular sources? I might be wrong though since its been a while since I've actually gone to a biology class. Not to mention it feels like something is weird with the question, so your probably entirely right.

 

[OzV]

"I'm not trying to be deliberately disagreeable since I know absolutely nothing about agriculture - but how did you come to that answer without using extra information? "

You can't. That's my point.

 

[gemmi]

hey can someone explain in simple terms exactly how punnet squares work? cos it confuses me when they ask for homozygous/heterozygous for particular genes and that sorta thing...

 

[rhia]

hey can someone explain in simple terms exactly how punnet squares work? cos it confuses me when they ask for homozygous/heterozygous for particular genes and that sorta thing...

the aim of a punnett square is to determine the probability as a percentage or ratio of an offspring's genotype.

to start with you need to know the genotypes of the parents. if the parent has a homozygous genotype, it means that the parent has identical alleles for a particular trait, e.g. AA. to be heterozygous means that the parent has two different alleles for a particular trait, e.g. Aa.

as far as i know, for the hsc course you're only required to construct a punnet square for a single trait.

so, they work by the construction of a 3x3 grid (the 1st square being redundant, or used to illustrate which squares respresent the mother and which the father) in which you enter the alleles of the mother on the top row, and the alleles of the father veritcally on the left hand side. then, you fill in the remaining four squares ... filling it in taking one allele from the mother, and one from the father.

these four squares represent the possible genotypes if the parents were to have four offspring, which can be used to calculate a percentage.

here is a dorky little animation, as it's difficult to explain without visuals: http://www.people.virginia.edu/~rjh9u/monohyb.html

here is another link which may be useful: http://en.wikipedia.org/wiki/Punnett_square

 

[gemmi]

thats real helpful. thanks!