A Punnett Square is a method that was originally invented by an early 20th century English geneticist called Reginald Punnett. A Punnett Square is used to predict the chances of offspring inheriting their parents’ genetic traits. In order to interpret Punnett Square diagrams, there are certain genetic terms that are involved. Provide your detailed explanations for the following terms:
The recessive allele for red hair is shown as ‘b’ and the dominant allele for black hair is shown as ‘B’. A woman who is homozygous for black hair marries a man who is homozygous recessive for red hair.
Using a Punnett Square, predict the hair colour outcome of their children. What’s the percentage outcome of them having a child with red hair or black hair? Explain your reasons.
ii) Some chickens can have black feathers, white feathers or a ‘checkered’ mixture of black and white feathers. The gene for the colour of their feathers is determined by a co-dominant genetic inheritance pattern. The allele for black feathers is B and the allele for white feathers is W. (1.1)
What must the genotype for black chickens be?
What must the genotype for white chickens be?
What must the genotype for checkered chickens be?
iii) If two ‘checkered’ chickens were crossed, what is the percentage probability of their chicks being black feathered, white feathered or ‘checkered’ feathered? Use a Punnett Square to support your answer and to give the resulting genotypes as well. (1.1, 1.2, 1.3)
If two characteristics are to be predicted in combination, then a dihybrid cross can be predicted using a Punnett Square and involves using the FOIL method to help determine the potential allele combinations that will be crossed in the Punnett Square. Refer to the AOLL pdf Information booklet for an explanation of the FOIL method as required.
In humans, the allele for being able to roll the tongue is dominant and is shown as ‘T’. It is dominant to the allele for not being able to roll the tongue, which is shown as ‘t’. Also, the allele for having unattached earlobes, shown as ‘U’ is dominant to the recessive allele for having attached earlobes, which is shown as ‘u’.
In the following couples, what would the ratio and percentage of the phenotypes be that can be predicted in their offspring? Produce a Punnett Square for each of the following couples and use the FOIL method to show how you arrive at your predicted phenotype outcomes. NB: Be very careful that your computer doesn’t alter your choice of upper or lower case letters in your Punnett Square. You may do these Punnett Squares by hand and photograph or scan them into your assignment if you’d rather.
i) Female partner: is heterozygous for being able to roll her tongue and homozygous recessive for having attached earlobes.
Male partner: is homozygous recessive for not being able to roll his tongue
and is homozygous recessive for having attached earlobes.
ii) Female partner: is homozygous recessive and cannot roll her tongue and is heterozygous for having unattached ear lobes.
Male partner: is homozygous recessive and cannot roll his tongue and is
also homozygous recessive for having attached earlobes.
Word count guideline:100 words
Question 4(1.2, 1.3, 2.1 monohybrid crosses)
Explain how, genetically, the sex of a baby is determined. Support your answer with a Punnett Square to show the predicted percentage outcomes for a male and female baby.
Word count guideline: 250 words
Question 5a (1.1 & 1.4 autosomal dominant)
Huntington’s disease is a late onset autosomal dominant genetic disorder that affects the central nervous system. This disease typically starts to develop when a person reaches their mid 30’s or 40’s, with no earlier signs or symptoms. The disease damages nerve cells in certain parts of the brain, causing progressive degeneration. The dominant allele can be shown as ‘H’; and the recessive allele as ‘h’.
The family tree diagram below shows the female partner who is heterozygous for Huntington’s Disease (Hh) and the male partner who is not affected and is homozygous recessive (hh). The female partner is therefore predisposed for Huntington’s Disease and although she has not yet developed the disease she is shown on the diagram as an ‘affected female’. This is because she will manifest the disease later in life. The diagram also shows the chances of the children of this couple being affected with Huntington’s Disease.
i) Give your explanation of how an autosomal dominant disease is inherited and the particular features of this form of inheritance pattern. (1.4)
ii) From the family tree diagram below, what is the genotype of the children that are predicted to inherit Huntington’s Disease? (1.1)
Word count guideline: 250 words
hh Hh
Question 5b (1.1 & 1.4 autosomal recessive)
Cystic fibrosis (CF) is an autosomal recessive inherited disease that affects the secretory glands, causing an excessive production of thick, sticky mucus and salty sweat. The most common effects of CF are progressive damage to the respiratory system and long-term digestive system problems. The normal dominant allele can be shown as ‘C’; and the mutant recessive allele as ‘c’.
The family tree diagram below shows the female partner who is heterozygous for Cystic Fibrosis (Cc) and the male partner who is not affected and is homozygous dominant (CC). The female partner doesn’t have any of the symptoms of the condition and is said to be a ‘carrier’ since she has one normal dominant and one mutant recessive allele in her genotype.
i) From the pedigree diagram below, what is the genotype of the third generation child marked as ‘X’? (1.1)
ii) Explain why the child marked as ‘X’ on the pedigree diagram below has inherited the cystic fibrosis disease whilst her parents that are carriers in the second generation are not affected. (1.4)
CC Cc
Word count guideline: 250 words
Question 6 (1.1 codominance, 2.2)
In mammals, genes located on the sex chromosomes are known as “sex-linked” genes.
The alleles for the colour of a cat’s fur coat are carried on the X sex chromosome.The alleles are ginger and black, but they are co-dominant.
The following Punnett Square diagram shows a black female cat mating with a ginger male cat.
Black female cat Alleles
Xá´®
Xá´®
Ginger
Male
Cat Alleles
Xá´³
Xá´®Xá´³
Xá´®Xá´³
Y
Xá´®Y
Xá´®Y
Using the above Punnett Square diagram to support your answer, explain the possible genotypes and phenotypes of coat colourings of their offspring, including an explanation of tortoiseshell coat colouring.
Word count guideline: 100 words
Question 7 (1.4, 2.2)
Haemophilia is an X-linked recessive inherited condition. Haemophilia affects the blood’s ability to clot and those affected don’t have as many clotting factors as normal. This means that people with haemophilia tend to bleed for longer than usual.
Of the couple in the image the mother is a carrier of haemophilia (XXh) and the father does not have haemophilia (XY).
a)
Draw a family tree showing this couple’s probable outcomes for their male and female offspring. Identify on the family tree the genotypes and percentage possibility of the offspring being carriers of haemophilia, being haemophiliac, or being free of the condition. (1.4)
b) Explain what an ‘x-linked recessive’ inheritance pattern entails, and provide examples from your family tree in part a) to support your points. (1.4 & 2.2)
Word count guideline: 250 words
Question 8 (3.1)
There are two main causes of heritable variation; continuous and discontinuous. Explain these two causes. Include in your explanation an account of recombination and mutation.
Word count guideline: 250 words
Question 9 (3.2)
Sometimes, though not a main cause, environmental factors can influence genetic variation.Classify and explain why you determine that the following variations given below are either caused entirely by genetic effects or caused by a combination of genetic and environmental effects. Place each variation into your chosen column in the table below and then give your ideas of why you have placed them there using references to support your ideas:
Obesity; eye colour; tallness; ability to sing; sex (male or female); masculinity; blood group; natural hair colour; sickle-cell anaemia; agility
Traits/variations caused entirely by genetic effects
Traits/variations caused by a combination of genetic & environmental effects
Citation (Name, Year) List fully at the end.
Why are these caused entirely by genetic effects?
Why are these caused by a combination of genetic and environmental effects & how are the phenotypes potentially altered?
Citation (Name, Year) List fully at the end.
Word count guideline: 300 words
Word Count 2500
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