genes

Question 1

D3.2.3: Genotype as the combination of alleles inherited by an organism.

Distinguish between gene and allele.

Define homozygous, heterozygous and hemizygous.

Answer 1

Genes are sections of DNA that code for a specific protein, and determine a trait.

Organisms which have cells with diploid nuclei will have two copies of each gene.
Homozygous individuals have two identical copies of an allele for a gene.
Heterozygous individuals have two different copies of an allele for a gene.

– An individual is a hemizygote when they only possess one allele for a given gene (males are hemizygous for all sex-linked traits)

Question 2

Compare and contrast different alleles of the same gene.

Answer 2

Dominant alleles are alleles that have the same effect on the phenotype in homozygous and heterozygous individuals.
The dominant allele is always expressed in the phenotype.

Recessive alleles are alleles that have an effect on the phenotype of a homozygous individual with two recessive alleles of the gene.
Recessive alleles are only expressed when an individual has two recessive alleles for a trait.

Question 3

D3.2.4: Phenotype as the observable traits of an organism resulting from genotype and environmental factors.

Distinguish between genotype and phenotype.

State a phenotype in humans that is due to genotype only.
State a phenotype in humans that is due to the environment only. (language spoken)
State a phenotype in humans that is due to the interaction of genotype and the environment. (athletic performance)

Answer 3

Genotype is the combination of alleles inherited by an organism.

Phenotypes are the observable traits of an organism determined by the genotype interacting with the environment.

Question 4

D3.2.6: Phenotypic plasticity as the capacity to develop traits suited to the environment experienced by an organism, by varying patterns of gene expression.

​Define phenotypic plasticity.
Outline an example of phenotypic plasticity.

Answer 4

Phenotypic plasticity is the capacity to develop traits suited to the environment experienced by an organism, by varying patterns of gene expression

Arctic foxes display phenotypic plasticity, meaning they can change their coat color and thickness in response to seasonal changes in snow and temperature.

Question 5

D3.2.8: Single-nucleotide polymorphisms and multiple alleles in gene pools.

State that new alleles of a gene are the result of mutation.
Define single-nucleotide polymorphism.

Explain why any number of alleles of a gene can exist in the gene pool but an individual only inherits two alleles.

Answer 5

A gene pool is all of the different genes and alleles present within an interbreeding population of a species.

Question 6

a2.2.6
prokaryote vs eukaryote chromosomes

Answer 6

• prokaryote: naked DNA, single chromosome, circular, found in nucleoid
• eukaryote: DNA w/ proteins, chromosomes in pairs, linear, found in nucleus

Question 7

Chromosomes can vary by…(LGP)

Answer 7

• length
• genes at specific loci
• position of centromere

Question 8

Homologous chromosomes similarities + differences

(formed after replication during interphase)

Question 9

D2.1.9: Meiosis as a reduction division

Explain what it means for chromosomes to be “homologous.”
Define diploid.
State the human cell diploid number.
Define haploid.
State the human cell haploid number.
List example haploid cells.

Answer 9

Cells and organisms that contain a homologous pair of each chromosome are called diploid (2n=46)

A cell or organism with a single copy of each chromosome is called haploid. (n=23)

Question 10

interphase

Answer 10

• duplicated DNA, DNA in chromatin form

Question 11

​Outline the events of prophase, metaphase, anaphase and telophase in meiosis I and meiosis II.

PROPHASE I

Answer 11

Homologous chromosomes pair up - The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other.

Crossing over - chromosomes swap pieces of DNA, increasing genetic variation (D2.1.11)

Question 12

METAPHASE I

Answer 12

Homologous chromosomes are independently aligned at the cell equator. Due to “independent assortment - ”, the orientation of each homologous pair is random and is not affected by the orientation of any other homologous pair (D3.2.16*)

Question 13

ANAPHASE I

Answer 13

Homologous pairs separate and each chromosome of the homologous pair moves towards opposite poles of the cells. Microtubules and microtubule motors move the chromosomes (D2.1.6)

Question 14

TELOPHASE I

Explain why meiosis I is a reductive division. (end up w/ haploid cells)
State that cells are haploid at the end of meiosis I.

Answer 14

New nuclei start to form around chromosomes at each pole
DNA uncoils from chromosome to form chromatin
Spindle fibers break apart
By the end of telophase I/cytokinesis there are TWO HAPLOID daughter cells. Each daughter cell has one copy of each chromosome type, either paternal or maternal.

Question 15

PMAT II - simple

At the end there are…

Answer 15

ANAPHASE II

Sister chromatids pull apart and move towards opposite poles of the cell. Microtubules and microtubule motors move the chromosomes (D2.1.6)

At the end of cytokinesis there are FOUR HAPLOID daughter cells.

Question 16

Given a diploid number (for example 2n=4), outline the movement and structure of DNA through the stages of meiosis.
Compare meiosis with mitosis.

Answer 16

Divisions
Ploidy
# of cells produced
Use (types of cells)

synapsis, independent assortment, crossing over only in meiosis I

Question 17

D2.1.4: Roles of mitosis and meiosis in eukaryotes.

State that meiosis is nuclear division that results in reduction of the chromosome number and diversity between genomes.

Outline the cause and consequence of anucleate cells (cells w/ no nucleus).

Answer 17

If cytokinesis occurs without prior nuclear division, one of the daughter cells will not have a nucleus - anucleate. Without DNA, anucleate cells, are unable to synthesize proteins, grow or maintain their structure.

Question 18

D3.2.11: Sex determination in humans and inheritance of genes on sex chromosomes.

Outline the structure and function of the two human sex chromosomes.
Explain why diploid cells have two copies of each autosomal gene. (inherit one from each parent)

Answer 18

The X chromosome is longer than the Y chromosome and contains many more genes.

The Y chromosome contains an SRY gene. If a human embryo has an SRY gene, it will develop male-typical characteristics.

The X chromosome does not contain an SRY gene. An embryo with two X chromosomes does not have an SRY gene, and develops female-typical characteristics.

Question 19

Outline gender determination by sex chromosomes. (sperm)

Answer 19

All female gametes, eggs, will have one X chromosome.
The male gametes, sperm, will have either the X or the Y chromosome.
Eggs fertilized by a sperm with an X chromosome will develop female-typical characteristics (XX).
Eggs fertilized by a sperm with a Y chromosome will develop male-typical characteristics (XY).

Question 20

A3.1.7: Karyotyping and karyograms. Application of skills: Be able to classify chromosomes by banding patterns, length and centromere position. Be able to evaluate the evidence for the hypothesis that chromosome 2 in humans arose from the fusion of chromosomes 12 and 13 with a shared primate ancestor.

Define karyotype and karyogram.
List the characteristics by which chromosomes are paired and arranged on the karyogram.
Define autosome and sex chromosome.

Describe the process of creating a karyogram.
Deduce the sex of a human individual given a karyogram.

Answer 20

A karyotype is the number and type of chromosomes found in a cell.
Humans have 22 pairs of autosomes, and 2 sex chromosomes.
A karyogram is a diagram or a photo of the chromosomes found in a cell.
In a karyogram, chromosomes are organized into homologous pairs of chromosomes.

Chromosomes in a karyogram are organized according to length/banding patterns/centromere position

Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

Question 21

D2.1.10: Down syndrome and non-disjunction

Define non-disjunction.
State the result of nondisjunction during anaphase I and anaphase II.
Describe the cause and symptoms of Down syndrome.

Answer 21

Non-disjunction is the failure of one or more pairs of homologous chromosomes or sister chromatids to separate fully during nuclear division.

Non-disjunction produces gametes with an extra chromosome or a missing chromosome.

If a gamete with an extra chromosome is fertilized by a normal gamete, the zygote, and resultant offspring, will have three copies of one chromosome.

Down Syndrome occurs when a person has three copies of chromosome 21.

Question 22

D2.1.11: Meiosis as a source of variation
Explain how meiosis leads to genetic variation in gametes.

Define bivalent = homologous pairs of chromosomes

Describe the process and result of crossing over during prophase I of meiosis.
Draw a diagram to illustrate the formation of new allele combinations as a result of crossing over.

recombinant chromosomes?

Answer 22

Two non-sister chromatids are brought together at the same point along their gene sequence.

The DNA strands of the two non-sister chromatids is cut and rejoined to the other chromatid. This results in an exchange of the DNA between the two chromatids (some genes are exchanged from one chromatid to another)

The location at which the fragments switch is called a chiasma (plural chiasmata). There can be multiple chiasmata along the chromosome.

Crossing over produces recombinant chromosomes, which are chromosomes with new combinations of alleles not present in either parent

Question 23

Describe the process and result of random orientation of bivalents during metaphase I of meiosis.
Draw a diagram to illustrate the formation of different chromosome combinations that result from random orientation during meiosis.
State that the number of chromosome combinations possible due to random orientation is 2^n.

Answer 23

Independent assortment results in gametes with a wide variety of maternal and paternal chromosome combinations.

there are 2^n possible combinations of maternal and paternal chromosomes that can be in the gametes formed during meiosis.

Question 24

D3.2.2: Methods for conducting genetic crosses in flowering plants

Define monohybrid, true breeding, hybrid, F1 and F2.

State an application of performing genetic crosses in plants. (make new crop varieties)

Answer 24

The offspring of the P generation are the F1 generation (first filial generation).
The offspring of two individuals from the F1 generation are the F2 generation (second filial generation).

A true breeding is a kind of breeding wherein the parents would produce offspring that would carry the same phenotype. This means that the parents are homozygous for every trait.

a monohybrid cross refers to a genetic cross involving parents that differ in only one trait, controlled by a single gene

a hybrid is an organism resulting from the cross-breeding of two different varieties, subspecies, species, or genera

Question 25

Outline the process of experimentally performing a genetic cross in flowering plants.

Answer 25

Gregor Mendel was an Austrian monk who developed the principles of inheritance by performing experiments on pea plants First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed) As a result of these experiments, Mendel discovered the following things: When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed E.g. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1 E.g. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1

Question 26

mendel's laws

Answer 26

Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene Law of Independent Assortment: The segregation of alleles for one gene occurs independently to that of any other gene* Principle of Dominance: Recessive alleles will be masked by dominant alleles†

Question 27

Codominance occurs Incomplete dominance compare and contrast - both don't follow mendel's princi[le of domincance - symbols; codomincance - alleles crossed for same gene

Answer 27

when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual. Heterozygotes therefore have an altered phenotype as the alleles are having a joint dominant effect. Incomplete dominance is a type of inheritance where the heterozygous phenotype is an intermediate blend of the homozygous phenotypes,

Question 28

blood groups

Answer 28

The A and B alleles are codominant and each modify the structure of the antigen to produce different variants. The O allele is recessive and does not modify the basic antigenic structure.

Question 29

D3.2.11: Sex determination in humans and inheritance of genes on sex chromosomes Define sex linkage Describe the pattern of inheritance for sex linked genes. Use correct notation for sex linked genes. Construct Punnett grids for sex linked crosses to predict the offspring genotype and phenotype ratios.

Answer 29

Sex linkage refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y) Hence, X-linked dominant traits are more common in females (as either allele may be dominant and cause disease). X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele. x-linked conditions Only females can be carriers (a heterozygote for a recessive disease condition), males cannot be heterozygous carriers. Males will always inherit an X-linked trait from their mother (they inherit a Y chromosome from their father). Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele).

Question 30

D3.2.12: Haemophilia as an example of sex-linked genetic disorder as well as red-green colourblindness Describe the cause and effect of hemophilia. Explain inheritance patterns of hemophilia.

Answer 30

- A recessive X-linked mutation - Haemophilia is a genetic disorder whereby the body’s ability to control blood clotting (and hence stop bleeding) is impaired The formation of a blood clot is controlled by a cascade of coagulation factors whose genes are located on the X chromosome When one of these factors becomes defective, fibrin formation is prevented - meaning bleeding continues for a long time - the patient can bleed to death.

Question 31

D3.2.13: Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases (Oxford Biology Course Companion page 181) Outline the conventions for constructing pedigree charts. Deduce inheritance patterns given a pedigree chart. **NOTE: A pedigree chart is a diagram that shows the occurrence and appearance of phenotypes of a particular gene or organism and its ancestors from one generation to the next.**

Question 32

SQUARE = CIRCLE = SHADED = UNSHADED =

Question 33

QUESTIONSSSSS 1.Does the trait skip generations? 2. Is the trait found much more often in males?

Question 34

X-LINKED DOMINANT

Answer 34

All daughters of a male who has the disease will also have the disease

Question 35

X-LINKED RECESSIVE

Answer 35

All daughters of a male who has the disease are diseased or heterozygous carriers.

Question 36

D3.2.7: Phenylketonuria as an example of a human disease due to a recessive allele Define “carrier” as related to genetic diseases = If a person has one allele for the genetic disease and one dominant allele, they will not show symptoms of the disease, but they can pass on the recessive allele to their offspring. Outline the genetic cause of phenylketonuria. List consequences of phenylketonuria if untreated. State how phenylketonuria is treated.

Answer 36

- pku IS AN autosomal recessive Phenylketonuria (PKU) is a recessive genetic disorder caused by mutations in the autosomal gene that codes for the enzyme phenylalanine hydroxylase (PAH). PAH catalyzes the conversion of the amino acid phenylalanine (found in all proteins) to tyrosine, but individuals with PKU are deficient in PAH, leading to the accumulation of phenylalanine levels in the body. High levels of phenylalanine can cause intellectual and developmental disabilities if not treated properly. The treatment consists of a diet containing little or no phenylalanine.

Question 37

D3.2.14: Continuous variation due to polygenic inheritance and/or environmental factors Explain polygenic inheritance using an example of a two gene cross with INCOMPLETE DOMINANT alleles. State that a normal distribution of variation is often the result of polygenic inheritance. State example human characteristics that are associated with polygenic inheritance. Outline two example environmental factors that can influence phenotypes.

Answer 37

each gene has two versions: an allele for increased melanin production (A,B,C) and an allele for reduced melanin production (a,b,c). Each dominant allele (A,B,C) in the genotype has an additive effect on the amount of melanin added to the skin. In this simple model, there are seven different shades of skin color ranging from very light (aabbcc) to very dark (AABBCC). - UV LIGHT/DIET

Question 38

D3.2.15: Box-and-whisker plots to represent data for a continuous variable such as student height Construct a Box-and-whisker plot Read and interpret a Box-and-whisker plot and the identify, describe and explain the patterns shown in your data