5.6 the difference of being a boy or girl

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• Middelbare school
• vwo
• Leerjaar 3

5.6 the difference of being a boy or girl

TB page 141

What is an allele?

What is a dominant allele?

What is a recessive allele?

male and female

male:    2x 22 autosomes + X + Y = 46 chromosomes

female: 2x 22 autosomes + X + X = 46 chromosomes

egg cell and sperm cell

egg cell contains 22 autosomes + X chromsome

sperm cell contains 22 autosomes + X or 22 autosomes + Y

today''s program 

learning objectives: you're able to answer the following questions

• why do certain inherited diseases occur more often in boys than in girls?


• given a certain combination of parents, how can you determine the genotype and phenotype of sex-linked inherited characteristics in the offspring? 

Sex-linked inheritance

• Female: XX,  male: XY
• Traits are only on the X chromosome
• On the Y are only the genes for male-ness
• Genotypes are written down like this (when they are sex linked!) :
  Female: X^HX^H, X^HX^h or X^hX^h 

     Male: X^HY or X^hY 

calculating chances

1. Write down both genotypes XX and XY
    with the trait X^HX^H and X^hY for example

2. Use a punnet square!

3. Calculate the chance of every possible phenotype and       
    answer the question

LET'S PRACTISE TOGETHER

A mother who is a carrier for colour-blindness gets a child with a father who has normal vision. 

What are the probabilities of their children being colour-blind? Use a Punnet square! 

colour-blindness is sex-linked!

question 41

start homework!

study 5.6

do excercise 41, 42, 43, 44, 45

Review: Sex-linked inheritance

• Female: XX,  male: XY
• Traits are only on the X chromosome
• On the Y are only the genes for male-ness
• Genotypes are written down like this (when they are sex linked!) :
  Female: X^HX^H, X^HX^h or X^hX^h 

     Male: X^HY or X^hY 

Let's compare..!

1. In a population of frogs, a certain disease (h) is recessive. Two heterozygous frogs are crossed. What are the phenotypic and genotypic ratios of their offspring?

2. In the same population of frogs, the same disease is now recessive and located on the X chromosome. A heterozygous carrier female frog is crossed with a male frog without the disease. What are the phenotypic and genotypic ratios of their offspring?

Compare and contrast

	Autosomal	Sex-linked
Expression in males/females	Males and females are equally likely to express the trait	Males are more likely to express recessive traits due to having an empty Y chromosome
Carrier status	Both males and females can be carriers	Only females can be carriers (heterozygous for the trait)

Sex-Linked Cross (Dominant Trait):

Question: 

In fruit flies, eye colour is sex-linked. Red eyes are dominant over white eyes. If a heterozygous red-eyed female mates with a white-eyed male, what are the possible genotypes and phenotypes of their offspring? Also give the %'s.

Sex-Linked Cross (Recessive Trait):

Question: 

In humans, color blindness is a recessive trait

carried on the X chromosome. If a carrier

female mates with a normal-vision male, what 

are the possible genotypes and phenotypes 

of their offspring? Also give the %'s.

Sex-Linked Cross

Question: 

In rabbits, black fur (B) is dominant over white fur (b), and the gene for fur color is located on the X chromosome. If a heterozygous black-furred female rabbit mates with a white-furred male rabbit, what are the possible genotypes and phenotypes of their offspring?

Homework

Workbook check:

• WB questions 43, 44, 45
• No new homework