16. Inheritance
A section of Biology, 9700
Listing 10 of 199 questions
Meiosis is an important process that contributes to genetic variation in a population. shows a pair of homologous chromosomes during prophase I of meiosis. On , use the letter C with a label line to identify where crossing over occurs. Describe how crossing over produces genetic variation in a population. Wing pattern in the butterfly species Heliconius melpomene is controlled by genes on autosomal chromosomes. The gene for banding pattern on the upper wing has two alleles: • a dominant allele coding for a full band • a recessive allele coding for a broken band. The gene for ray pattern on the lower wing has two alleles: • a dominant allele coding for rays • a recessive allele coding for no rays. Scientists crossed a butterfly that was homozygous dominant for both genes with a butterfly that was homozygous recessive for both genes. The scientists wanted to check whether the phenotypic ratio for offspring in the F2 generation agreed with the expected phenotypic ratio of 9 : 3 : 3 : 1. The results of this genetic cross are shown in . × rays parental generation F1 generation F2 generation number of offspring of each phenotype full band no rays broken band Explain the term F1 generation. The scientists used the chi-squared ( χ 2) test to compare their data to the expected phenotypic ratio of 9 : 3 : 3 : 1. The formula for the chi-squared test is shown in . χ 2 = ! (O – E)2 E key to symbols O = observed value E = expected value Complete Table 4.1 and use the chi-squared formula in to calculate the χ 2 value for these data. Table 4.1 phenotype expected ratio O E O – E (O – E)2 (O – E)2 E full band and rays 76.5 22.5 506.25 6.62 full band and no rays 25.5 – 4.5 20.25 0.79 broken band and rays broken band and no rays 8.5 – 0.5 0.25 0.03 χ 2 = The critical value at the 0.05 probability level and three degrees of freedom is 7.81. Using the result of your calculation in , explain whether the results of the study agree with the expected ratio of phenotypes for the F2 generation. Suggest two reasons why phenotypic ratios in the F2 generation do not always match the expected ratios.
9700_m22_qp_42
THEORY
2022
Paper 4, Variant 2
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Outline the technique of in-vitro fertilisation (IVF). For IVF to be successful, a sperm must have an undamaged plasma (cell surface) membrane, an intact acrosome (a sperm’s large lysosome) and be capable of producing ATP for movement. One method of assessing the quality of a sample of sperm is to mix it with three chemical probes that bind to specific components of the sperm. The probes fluoresce when the sperm are examined with a microscope using ultra-violet (Ulight, allowing their uptake to be determined. The three probes fluoresce with different colours. • Probe 1 combines with DNA and fluoresces red, but can enter a sperm only when its plasma membrane is damaged. • Probe 2 combines with sugars in the acrosome and fluoresces yellow, but can enter the acrosome only when the acrosome membrane is damaged. • Probe 3 combines with mitochondria and fluoresces bright green in sperm with active mitochondria and less brightly when the mitochondria are less active. A sample of sperm was mixed with all three probes and examined using UV light. Complete Table 3.1 by placing ticks (3) in the appropriate boxes to describe the appearance of sperm that would be suitable for use in IVF. Table 3.1 appearance of sperm suitable for use in IVF target of probe red yellow green colourless DNA acrosome mitochondria The technique of intracytoplasmic sperm injection (ICSinvolves injecting a single, chosen sperm into an oocyte. This technique is often used when standard IVF has failed. Researchers in Hawaii think that the acrosome of the sperm should be removed before the sperm is injected into the oocyte. Suggest one reason why it might improve the success rate of ICSI to remove the acrosome before injecting a sperm into an oocyte.
9700_s11_qp_41
THEORY
2011
Paper 4, Variant 1
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The cells in are from the same organism and look the same. The cells in have been produced by mitosis and the cells in have been produced by meiosis. Complete the table to show two differences between cells that have been produced by mitosis compared to cells that have been produced by meiosis. mitosis meiosis Explain why the organism produces cells by meiosis. is a transmission electron micrograph of a dividing cancer cell. X Y magnification x 3000 Calculate the actual width of the cell shown in at X-Y. Show your working and give your answer to the nearest micrometre (μm). answer μm The cancer cell shown in has more mitochondria and rough endoplasmic reticula (RER) compared to the non-cancerous cell from which it originated. Suggest why this is so.
9700_s13_qp_22
THEORY
2013
Paper 2, Variant 2
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shows some of the steps involved in in-vitro fertilisation (IVF). Step 1 egg production stimulated Step 3 sperm sample provided Step 2 eggs retrieved from ovary Step 5 embryos introduced into uterus Step 4 eggs and sperm combined to allow fertilisation Explain how egg production is stimulated at step 1. Following step 3 in , the sperm sample is placed in a solution containing various nutrients and other substances, for up to one hour, before being added to the eggs. Explain why this is done. In 2010, researchers found that they could predict with 93% certainty which embryos produced by in-vitro fertilisation would develop into healthy babies when implanted into the uterus. Their technique involved the use of time-lapse microscopy. The successful embryos met three criteria: • the first cytokinesis lasted between 0 and 33 minutes • the time interval between the first and second cell division was between 7.8 and 14.3 hours • the time interval between the second and third cell division was between 0 and 5.8 hours. Suggest one advantage of the use of this new technique in the IVF procedure. Suggest one disadvantage of the use of this technique.
9700_s13_qp_42
THEORY
2013
Paper 4, Variant 2
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The sex chromosomes of domestic chickens are known as W and Z. • Male chickens have two Z chromosomes. • Female chickens have one W and one Z chromosome. A gene for feather colour in chickens is located on an autosome. This gene (C) has two alleles • black (CB) • splashed-white (CS). When a chicken with black feathers is mated with a chicken with splashed-white feathers, all the offspring will have blue feathers. A gene for feather pattern is located on the Z chromosome but not on the W chromosome. This gene has two alleles • barred feathers (A) • non-barred feathers . State the type of inheritance for feather colour and for feather pattern. feather colour feather pattern A male chicken with black, non-barred feathers was crossed with a female chicken with splashed-white, barred feathers. All the offspring had blue feathers, but the males were barred and the females were non- barred. Using the symbols given, draw a genetic diagram to show this cross. parents phenotype male, black, non-barred feathers female, splashed-white, barred feathers parents genotype gametes offspring genotypes offspring phenotypes male, blue, barred feathers female, blue, non-barred feathers Explain how the genotype of a male chicken with blue, barred feathers could be determined.
9700_s16_qp_42
THEORY
2016
Paper 4, Variant 2
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The red poppy, Papaver rhoeas, and several species of daisy of the family Compositae often co-exist as weeds of wheat fields. shows changes in the percentage frequency of red poppies and daisies in an area of wheat fields over a six year period from 1998 to 2003. From 1985, the herbicide metsulfuron-methyl was used to control weeds in this area of wheat fields. This practice continued throughout the six year period. 1998 showed the first occurrence of a red poppy known as biotype X. This red poppy had a specific mutation not present in normal red poppies. year percentage frequency daisies normal red poppy biotype X red poppy Describe how the percentage frequencies of daisies and red poppies changed over the six year period. Metsulfuron-methyl acts by inhibiting an enzyme called acetolactate synthetase, which is needed for the daisies and red poppies to synthesise three amino acids essential for growth. The specific mutation carried by the red poppies of biotype X occurred within the gene coding for this enzyme. The mutation changed amino acid 197 of acetolactate synthetase from proline to leucine. Suggest the effect of this mutation on the structure and activity of the acetolactate synthetase enzyme of biotype X red poppies. Suggest the effect of this mutation on the biotype X red poppies in the presence of metsulfuron-methyl. With reference to , predict and explain the effect of biotype X red poppies on the relative proportions of weeds and wheat in the area of wheat fields in 2003 compared to 1998. . . Suggest how stopping the use of the herbicide metsulfuron-methyl and replacing it with a herbicide that inhibits a different target enzyme in weeds would affect the abundance of red poppies of biotype X.
9700_s17_qp_41
THEORY
2017
Paper 4, Variant 1
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Questions Discovered
199