topic 3 - genetics

state advantages of asexual reproduction

• populations can be increased rapidly

• can exploit suitable environments quickly

• more time and energy efficient

• reproduction completed much quicker than sexual reproduction

• don’t need to waste time finding a mate

state disadvantages of asexual reproduction

• limited genetic variation within populations

• population is vulnerable to changes in condition (may only be suited for one habitat)

• disease is likely to affect the whole population

state advantages of sexual reproduction

• increases genetic variation

• species can adapt to new environments due to variation

• disease is less likely to affect the whole population

state disadvantages of sexual reproduction

• takes time and energy to find a mate

• difficult for isolated members of the population to reproduce

describe meiosis

• cell division

• producing four daughter cells

• each with half the number of chromosomes

• resulting in the formation of genetically-different haploid gametes

state what a DNA polymer is made of

• two strands coiled

• to form a double helix

• linked by a series of complementary base pairs

• joined together by weak hydrogen bonds and nucleotides

• that consist of a sugar and phosphate group

• with one of the four different bases attached to the sugar

state complementary DNA base pairs

• A+T

• G+C

genome definition

entire DNA of an organism

gene definition

• section of a DNA molecule

• that codes for a specific protein

explain how DNA can be extracted from fruit

1. mash the fruit and mix into a beaker containing a solution of detergent and salt

2. filter the mixture into a test tube

3. gently add ice-cold ethanol to the filtrate by pouring slowly down the side of the test tube

4. DNA will appear as a stringy, white precipitate

5. the precipitate can be extracted using a glass rod

explain why detergent is used in the extraction of fruit DNA

• breaks down the cell membranes

• causing fruit cells to release their DNA

explain why salt is used in the extraction of fruit DNA

salt causes the fruit DNA to stick together

explain why ice-cold ethanol is used in the extraction of fruit DNA

ethanol causes the DNA to precipitate

explain how order of bases in a gene decides the order of amino acids in the protein

• each gene acts as a code for making a specific protein

• gene = triplet of bases

• amino acids are made in the number and order

• dictated by the number and order of base triplets

• amino acid molecules join together in a long chain to make a protein molecule

• proteins are then folded into their correct shape

• to make them functional

describe the stages of protein synthesis (transcription)

• occurs in the nucleus of the cell

• part of a DNA molecule unwinds

• when hydrogen bonds between complementary base pair breaks

• this exposes the gene to be transcribed

• RNA polymerase binds to a region of non-coding DNA in front of the gene

• RNA polymerase makes a complementary copy of the code from the gene

• by building mRNA

• mRNA leaves the nucleus via a pore in the nuclear envelope

describe the stages of protein synthesis (translation)

• occurs in the cytoplasm of the cell

• after leaving the nucleus, mRNA attaches to a ribosome

• cytoplasm contains free molecues of tRNA

• tRNA have anticodons at one end

• and a region where a specific amino acid can attach at the other

• tRNA bind with their specific amino acid and bring them to the mRNA on the ribosome

• anticodons on tRNA pair with complementary codons on the mRNA

• 2 tRNA fit onto the ribosome at one time, bringing the amino acid they are each carrying side by side

• a peptide bond is formed between the 2 amino acids (polypeptide)

• process continues until a ‘stop’ codon on the mRNA is reached

• this acts as a signal for translation to stop

• and the amino acid chain is complete

• the chain will then fold into a protein

state what anticodons are

a triplet of unpaired bases

explain the role of ribosomes in protein synthesis

• ribosomes read the code on the mRNA in groups of 3

• each triplet of bases on the mRNA code for a specific amino acid

• ribosomes translate the sequence of bases

• into a sequence of amino acids

state what RNA polymerase is

enzyme

state what mRNA is

single-stranded nucleic acid molecule

explain how genetic variation in non-coding region of a gene can affect phenotype

• RNA polymerase attaches to a non-coding section of DNA

• if this region has a mutation, the ability of RNA polymerase to bind there is affected

• causing less mRNA to be transcribed

• and less protein that the gene codes for will be synthesised

• phenotype is affected by how much protein is produced

• even if the coding region of DNA is completely normal

explain how genetic variation in coding region of a gene can affect phenotype

• mutations in the coding region of DNA cause the gene to code for

• a different sequence of amino acids

• amino acid sequences are highly specific

• and form specific shapes and types of proteins

• change in sequence of amino acids may affect the shape of the protein

• and thus change its function

• meaning a change in phenotype can be caused

describe gregor mendel’s work

• mendel studied how characteristics were passed on between generations of plants

• in his first experiment, he crossed a tall pea plant with a dwarf pea plant

• in his second experiment, he crossed two tall pea plant offspring

• he observed that the inheritance of each characteristic is determined by

• ‘hereditary units’ that are passed onto descendants unchanged

• mendel discovered that the tall unit in pea plants was dominant

describe gregor mendel’s conclusions

• characteristics are determined by hereditary units

• and hereditary units are passed unchanged from parents to offspring

• offspring receive one hereditary unit from each parent

• hereditary units can be dominant or recessive

explain why understanding inheritance before mendel’s work was difficult

• DNA, genes and chromosomes

• had not been discovered yet

explain why there are differences in inherited characteristics as a result of alleles

• alleles are different version of the same genes

• difference in inherited characteristics arise due to variation in alleles

chromosome definition

• thread-like structure of DNA

• carrying genetic information

• in the form of genes

• located in the nucleus

allele definition

different versions of the same gene

dominant definition

• allele that is always expressed

• if at least one copy is present

recessive definition

• allele only expressed

• only if two copies are present

homozygous definition

if two alleles are the same

heterozygous definition

if two alleles are different

genotype definition

• combination of alleles

• that control each characteristic

phenotype definition

observable characteristics

gamete definition

sex cells

zygote definition

fertilised egg cell (ovum)

explain monohybrid inheritance

• inheritance of characteristics

• controlled by a single gene

state how monohybrid inheritance can be investigated

• punnett squares

• family pedigrees

explain how punnett squares work

• genetic diagram that shows

• the possible combinations of alleles

• that could be produced in offspring

• ratios of combinations can be worked out from this

explain how family pedigrees work

• trace the pattern of inheritance

• of a specific phenotype/genotype

• through generations of a family

• can be used to work out the probability that a family member will inherit a genetic disorder

describe how the sex of offspring is determined at fertilisation using genetic diagrams

• sex is determined by an entire chromosome pair

• females have XX sex chromosomes

• males have XY sex chromosomes

• only a male can pass on a Y chromosome

• so the male is responsible for determining the sex of the child

• punnett squares of the parents chromosome pair can be used

• to determine the ratio of the offspring’s sex

describe the inheritance of the ABO blood groups

• inheritance of blood groups is an example of codominance

• three alleles determine blood group inheritance instead of two

• alleles I^A and I^B are both codominant

• they are both dominant to I^O

• I^A results in production of antigen A in the blood

• I^B results in production of antigen B in the blood

• I^O results in no antigens being produced in the blood

codominance definition

both alleles within a genotype are expressed

state what phenotype I^AI^A produces

A

state state what phenotype I^AI^O produces

A

state what phenotype I^BI^B produces

B

state what phenotype I^BI^O produces

B

state what phenotype I^AI^B produces

AB

state what phenotype I^OI^O produces

O

explain how sex-linked genetic disorders are inherited

• sex-linked genetic disorders are usually caused by an X chromosome allele

• males are more likely to be affected by X-linked recessive disorders

• as they do not have a dominant X chromosome to mask the affects

• a female with a masked recessive allele is a carrier

• and has a 50% chance of passing it to her offspring

• if the offspring is male, he’ll have the disease

what are most phenotypic features a result of

• multiple genes

• rather than single gene inheritance

describe the influence of genetic variation on phenotype

• meiosis creates genetic variation between gametes

• meaning each gamete carries significantly different alleles

• during fertilisation, any male gamete can fuse with a female gamete

• to form a zygote

• random fusion of gametes at fertilisation also creates genetic variation

• between zygotes

• zygotes eventually grow and develop into adults

• with differing expressed phenotypes

describe the influence of environmental variation on phenotype

• environmental factors such as

• accidents, climate, diet, culture and lifestyle

• can cause differences in phenotypes such as

• scarring (accidents), eating too much (weight gain),

• raised in a different country (different language)

• plant in shade (grows taller)

• which are acquired characteristics

state the outcomes of the human genome project

• mapped and identified

• all the genes in the human genome

state the applications of the human genome project within medicine

• information about DNA can be helpful

• for forensic science

• tracing human migration patterns

• and understanding and treating genetic disorders

state what causes genetic variation

genetic mutation

state the degree of variation within a population of a species

extensive

state the effect most genetic mutations have on phenotype

none

state the effect some genetic mutations have on phenotype

small

state the effect few genetic mutations have on phenotype

significant