biology - genetics (3.1, 3.2, 3.11 - 3.23)

3.1 advantages of asexual reproduction

lack of need to find a mate - more time & energy efficient

rapid reproductive cycle

3.1 disadvantages of asexual reproduction

no variation in population - species only suited to one habitat, disease may effect all individuals in population

3.2 advantages of sexual reproduction

variation in population - survival advantage as species can adapt to environmental changes, disease less likely to affect all individuals in population

3.2 disadvantages of sexual reproduction

requirement to find a mate - uses time & energy, isolated individual cannot reproduce (extinction)

slower reproductive cycle

asexual reproduction

mitosis, 1 parent

sexual reproduction

meiosis, 2 parents, fusion of male & female gametes

3.11 how did Mendel observe characteristic in pea plants?

bred pea plants together using a paintbrush to move pollen (containing male gametes) from one plant to flower of another

bag placed over flower on plant & sealed (no cross-contamination)

planted seeds that formed & observed characteristics of offspring

3.11 Mendel’s conclusions

concluded inherited ‘factors’ control variation of characteristics

factors exist in different versions (alleles) that do not change

plant has 2 factors for each characteristics (either same version/2 different versions)

plants with 2 factors of same version: true-breeding; if plant was self-pollinated, offspring all had same variation as parent

3.11 Mendel’s 3 laws of inheritance

each gamete receives only one factor for a characteristic

the version of a factor a gamete receives is random & doesn’t depend on other factors in gamete

some versions of a factor are more powerful than other & always have an effect in offspring

3.11 difficulties understanding inheritance

Mendel’s work mostly not accepted

scientists didn’t understand how ‘factors’ could explain the many variations in characteristics like human eye colour

also how his ideas could explain Darwin’s theory of evolution - argued if factors couldn’t change, species couldn’t change/evolve

once chromosomes discovered (1880s) scientists started understanding how Mendel’s factors could work

3.12 why are there differences in inherited characteristics?

there’s 2 copies of every chromosome in a body cell nucleus

so a body cell contains 2 copies of every gene

each copy of a gene may be a different allele

different combination of alleles in each person gives everyone slightly different characteristics (genetic variation)

3.13 chromosome

long coiled molecule of DNA carrying genetic information in the form of genes

3.13 gene

section of DNA on a chromosome that codes for a particular protein

3.13 allele

different forms of the same gene

3.13 dominant

a dominant allele is always expressed, capital letter

3.13 recessive

a recessive allele is only expressed if two copies are present (no dominant allele), lowercase letter

3.13 homozygous

both alleles for one gene are the same

3.13 heterozygous

both alleles for one gene are different

3.13 genotype

organism’s genetic composition - combination of alleles that control each characteristic

3.13 phenotype

organism’s observable characteristics

3.13 gamete

sex cells (sperm & egg) with 23 chromosomes

3.13 zygote

diploid cell formed by fusion of nucleus of male gamete with nucleus of female gamete

monohybrid inheritance

the inheritance of one gene

3.14 monohybrid inheritance - genetic diagrams

3.14 monohybrid inheritance - Punnett squares

3.14 monohybrid inheritance - family pedigrees

3.15 how is the sex of offspring determined at fertilisation?

3.16 Calculating probabilities, ratios & % for dominant & recessive traits

4 different blood groups

A, B, AB, O

how is blood group determined?

‘marker molecules’ on outside of RBCs

3 main types of markers: A, B, O

3.17 inheritance of ABO blood groups

gene responsible for markers in ABO system has 3 alleles: I^A, I^B, I^O

everyone has 2 copies of the gene, so can be: homozygous for any of the 3 alleles; heterozygous for any 2 of the 3 alleles

person with genotype I^AI^B shows effects of both alleles so has blood group AB

3.17 codominance

when both alleles for a gene affect the phenotype (e.g. AB blood group)

3.18 how are sex-linked genetic disorders inherited?

chromosomes in diploid cells come in pairs

in most pairs chromosomes have same genes

however human Y sex chromosome is missing some genes found on X chromosome

so man (XY) will have only one allele for some genes on X chromosome (because those genes are missing on Y chromosome)

if allele for one of these X chromosome genes causes a genetic disorder, man will develop disorder

if woman inherits ‘disorder’ allele, she may have ‘healthy’ allele on other X chromosome

if ‘disorder’ allele is recessive, won’t get disorder

if she inherits 2 recessive alleles, will get disorder

probability woman getting disorder < probability man getting disorder

3.19 what are phenotypic features a result of?

most phenotypic features are the result of multiple genes rather than single gene inheritance (e.g. several genes affect eye colour in humans)

mutation

a change in a gene that creates a new allele

3.20 causes of variation that influence phenotype - genetic variation

different characteristics as a result of mutation & sexual reproduction

different alleles inherited during sexual reproduction

different alleles are produced by mutations (some cause changes in phenotype)

3.20 causes of variation that influence phenotype - environmental variation

different characteristics caused by an organism’s environment (acquired characteristics) - e.g. loss of limb

continuous variation

data can be any value in a range - e.g. leaf length

discontinuous variation

data can only take a limited set of values - e.g. number of whole leaves

3.21 Human Genome Project - outcomes & potential applications in medicine

produced map of complementary base pairs in 1 set of 46 human chromosomes

found many sections of DNA that are the genes

mapped other human genomes

showed there’s variations between people, but over 99% of DNA bases in different people are the same

indicates people’s risk of developing diseases caused by different alleles of genes

helps identify which medicines might be best to treat person’s illness - alleles we have can affect how medicines work in body

3.22 genetic variation in population of species

usually extensive genetic variation within population of species & these arise through mutations

3.23 effect of mutations on phenotype

most genetic mutations have no effect on phenotype

some mutations have small effect on phenotype

rarely single mutation will significantly affect phenotype