Science tests

Page 1

QUESTION ONE: DNA AND VARIATION

Genes determine many of the features of organisms, such as the colour of the flowers on a plant. A gene is a part of a DNA molecule.

The type of plant shown in the photograph below can have red or white flowers.

Red flowers are due to a dominant allele and white flowers are due to a recessive allele.

Discuss how information in DNA gives organisms (like the plant above) their individual features, such as red and white flowers. In your answer, you should:

explain the difference between a gene and an allele

draw a labelled diagram in the box on page 3 to show the relationship between a gene and an allele and the structure of a DNA molecule

explain how the base sequence on DNA determines a particular feature (eg, flower colour) and different forms (variations) of that feature (eg, red and white flowers).

A gene is a section of DNA that codes for a particular protein. The protein that the gene codes for determines a feature in the body. An allele is a different form of a gene. A difference in base sequence (alleles) code for different features of the same trait such as red or white flowers on this type of plant. For example, a gene may be flower colour wheras the alleles may be red flower colour and white flower colour. Two alleles form a genotype and depending on the infomation (homozygous recessive hh, homozygous dominant HH or heterozygous Hh) will determine the phenotype shown.

DNA (Deoxyribonucleic acid) is a double helix which looks like a twisted ladder. DNA is made up of sugar molecules, phosophate groups and bases(Adenine, thymine, cytosine, guanine) These form a nucleotide. Three nucleotides form nucleotide tripletes which code for an amino acid. Different combinations of amino acids code for a particular protein depending on the sequence of base pairs. Adenine always pairs with thymine, and cytosine always pairs with guanine. (unless there is a mutation in the base sequence) Depending on the sequence of base pairs in DNA therefore determines a specif protein to be made. A specific protein forms a specific gene. for example flower colour. The sequence of bases will depend what features/alleles that are present. For example red flowers will have a different base sequence to white flowers for that gene of flower colour.

QUESTION TWO: SEXUAL REPRODUCTION

Meiosis is a particular form of cell division that produces male and female gametes.

Describe what gametes are and explain why they are needed for sexual reproduction.

Gametes are sex cells produced by meiosis. Sperm arethe gametes produced by males and eggs are the gametes produced by females. These are found in the reproductive organs. eg testes and ovaries. Gametes contain haploid number of chromosomes(Half the numer of a full set of chromosomes) and have a unique mixture of genetic infomation. They are neeed for sexual reproduction because when fertilization happens, The sperm and egg fuse together to form a zygote with diploid number of chromosomes (full set of chromosomes), with a unique mixture of genetic code. The haploid sperm and haploid egg have half the genetic infomation, and when they fuse, their is a full set of chromosomes, half paternal, and half maternal. If this didn't happen or zygotes had diploid number of chromosomes, then when the gametes fuse together, the zygote formed would have double the number of diploid chromosomes. The zygote would not be able to survive and function at all.

Meiosis contributes to genetic variation.

Discuss how meiosis contributes to genetic variation, and why genetic variation is important in a population. In your answer, you should:

describe what is meant by genetic variation

explain how the process of meiosis leads to genetic variation

explain why genetic variation is of benefit to a population.

You may draw labelled diagrams in the box on page 5 to support your answer.

Genetic variatio in species is where sexual organisms have different genetic infomation to eachother to ensure survival of that species. There is variation due to meiosis. Meiosis is the cell division to form gametes. In meiosis, there are two divisions. The first is where the chromosomes replicate in their homologous pairs and line up in the middle of the nucleotide

QUESTION THREE: MONOHYBRID CROSSES

The allele for a cleft chin (D) is dominant over the allele for a smooth chin (d).

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The pedigree diagram below shows the chin types in a family.

Explain how evidence in the pedigree diagram on page 6 shows that the cleft chin allele (D) is dominant over the smooth chin allele (d).

In your answer, you should:

explain what the term dominant allele means

draw a Punnett square(s) to show your reasoning.

Explain why the genotype of male A in generation 2 of the pedigree diagram shown on page 6 must have the genotype Dd.

Individuals A and B, as shown on the pedigree diagram on page 6, decide to have another child. They draw a Punnett square to find what type of chin their child might have.

Explain why the result predicted by the Punnett square may not accurately tell them what type of chin their child will have.

In your answer, you should:

draw a Punnett square to show the cross between individual A and individual B

determine the probabilities of the child having a cleft chin and having a smooth chin

explain why the ratio of children born into the family with cleft and smooth chins may not match the probabilities.

QUESTION FOUR: SURVIVAL

Mutations can occur in DNA during cell division.

Explain whether a mutation could be inherited if it occurred in a skin cell of an individual. In your answer, you should:

describe what a mutation is

explain what determines whether a mutation is able to be inherited or not.

A population of plants, species A, living in a certain area shows a lot of variation in its leaf size, from very small to very large as shown in the graph below.

Leaf size of plants of species A

Leaf size affects the ability of a plant to absorb sunlight and make food. Plants with larger leaves can live in areas with lower light levels.

A new plant, species B, starts growing in the same area as species A. Species B plants grow taller than species A plants, which reduces the light available to plants growing below species B.

Discuss how variation in leaf size occurs in the starting population of species A and explain how this might help species A to survive when species B starts growing in the same area.

In your answer, you should consider:

what causes variation within a population

the effect of reduced light on different individuals of plant species A.

Extra paper for continuing your answers, if required.Clearly number the question.

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Extra paper for continuing your answers, if required.Clearly number the question.

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1.9 genetic variation