B6 X and Y Chromosomes (page 71) Flashcards
What do your chromosomes control?
whether you’re Male of Female.
How many pairs of Chromosmes are there in every juman body cell?
23 pairs (see page 15)
How many of the 23 chromosomes are Matched Pairs of Chromosomes?
22 pairs are matched pairs, these just control characteristics.
Why is the 23rd pair of chromosome labelled XY or XX?
They’re the two chromosomes that decide your sex - whether you turn out Male or Female.
What chromosome does Males have?
Males have an X and a Y chromosome: XY
The Y chromosome causes Male characteristics.
What chromosome does Females have?
Femails have two X chromosomes: XX
The XX combination allows female characteristics to develop.
What happens when making Sperm with the chromosomes?
The X and Y chromosomes are drawn apart in the first division in Meiosis (see page 70).
What is the percentage that each sperm cell gets an X-chromosome or a Y-chromosome during Meiosis?
There is a 50% chance each sperm cell gets an X-chromosome and a 50% chance it gets a Y-chromosome.
What happens during Meiosis when making eggs from a female?
Is a similar thing as making sperm, but the orginal cell has two X-chromosomes, so all the Eggs have One X-chromosomes.
What do genetic diagrams show?
it shows the possible Gamete Combinations.
How do you find the probability of getting a boy or a girl?
you can draw a genetic diagram.
What are Genetic diagrams?
they are just models that are used to show all the possible genetic outcomes when you cross together different genes or chromosomes.
How do you work out the genetic diagram to find the probability of getting a boy or a girl?
Put the possible gametes (eggs or sperm) from one parent down the side, and those from the other paretn along the top
Ten in each middle square you fill in the letters from the top and side that line up with that square. The pairs of letters in the middle how the possible combinations of the gametes
There are two XX results and two XY results, so there’s the same proability of getting a boy or a girl.
Dont forget that this is 50:50 ratio is only a probability at each pregnancy.
(see the diagram on page 71 - it is called a ‘Punnett Square’
There is another type of genetic diagram, but it looks a bit more complicated, but it shows exactly the same thing. describe the genetic diagram on page 71?
1) at the top are the parents.
2) the middle circles show the possible gametes that are formed. One gamete from the female combines with one gamete from the male (during fertilisation).
3) The criss-cross lines show all the possible ways the X and Y chromosomes could combine. The possible combinations of the offspring are shown in the bottom circles.
4) Remember, only one of these possibilities would actually happen for any one offspring
(most genetic diagrams you’ll see in exams concentrate on a gene instead of a chromosome. But the principle’s the same)
What combination of sex chromosomes do human females have? (1 mark)
XX (1 mark)
