P2B - Inheritance 1

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Gametes in animals

• ova and sperm (much smaller)

• not paired = contain 23 single chromosomes

• made by type of cell division called meiosis

Gametes in certain plants

• made by meiosis = produces non-identical cells

• happens in flowering plants - gametes are pollen and egg cells

Sexual reproduction in animals and flowering plants

• fusion of male and female gametes = fertilisation

• mixing of genetic info = offspring has DNA from both parents

• since every gamete is different, theres variation in offspring

Asexual reproduction

• only one parent = no mixing of DNA

• doesn't involve gametes = only involves mitosis, not meiosis

• offspring are all genetically identical = Clones

• seen in aphids and certain plants that grow buds which fall off

v1 - sexual and asexual reproduction

Meiosis

• only happens in reproductive organs, humans = testes and ovaries

• produces 4 gametes from 1 cell - each gamete is genetically diff from another (has diff alleles)

1. All chromosomes are copied

2. Cell divides into 2

3. Both divide again forming gametes with single chromosomes (half the number of chromosomes)

Fertilisation

• in sexual reproduction male and female gametes fuse

• after this fertilisation, the cell now has the normal number of chromosomes

After fertilisation

• cell divides by mitosis producing a clump of identical cells = Embryo

• As embryo develops, cells differentiate forming different cell types

  • in animals; nerve and muscle cells

v2 - meiosis and fertilisation

Adv of sexual reproduction

• Variation in offspring

• Species can adapt to new environments due to variation = some offspring survive = species survival advantage by natural selection

• Humans take advantage of this variation when doing selective breeding for crops (wheat - high yield, good quality)

• A disease is less likely to affect all the individuals in a population

Adv of asexual reproduction

• one parent needed

• No need to find a mate = more efficient in time and energy

• Faster than sexual

• Fast = very useful when conditions are favourable. Organism produces many genetically identical offspring rapidly

Dis of asexual reproduction

• Since genetically identical, theres a risk of all offspring dying if conditions become unfavourable

• species may be only suited to one habitat

• no variation in a population

Dis of sexual reproduction

• time and energy needed to find a mate

• not possible for isolated individual

Malaria parasite - its type of reproduction

• In human - asexually

• In mosquito - sexually

Fungi - its type of reproduction

• Asexually by producing spores

• Sexually to generate variation in offspring

Flowering plants - its type of reproduction

• Sexually to produce seeds

• Strawberry plants can also reproduce asexually by sending out runners

• Or asexually with daffodils when their bulbs divide

v3 - adv and dis of sexual / asexual reproduction

DNA

• Molecule that makes up chromosomes

• It’s genetic material because it determines our inherited features

• consists of 2 strands, each are polymers that wrap around each other to form a double helix

Genes

• A gene is a small section of DNA on a chromosome

• Each gene encodes for a specific sequence of amino acids to make a specific protein

• Humans have thousands, e.g chromosome 9 has over 700 diff genes

• Chromosome pairs both have the same genes

Genome

• is the entire genetic material of an organism

Benefits of understanding the human genome

• search for genes that are linked to a disease (genes increasing cancer or alzheimer’s disease risk)

• understand and treat inherited disorders (cystic fibrosis)

• trace human migration patterns from the past = discover peoples ancestry

v4 - DNA and the genome

DNA in detail

DNA is a double stranded polymer of molecules called nucleotides:

• Phosphate group attached to sugar molecule (pentagon) which is attached to a molecule called a base

• Phosphate and sugar group never change but bases do

• Theres are 4 bases - ATCG = DNA has 4 different nucleotides

• Same bases always pair on the opposite strands = A&T, C&G = DNA strands are complementary

v5 - DNA structure

how DNA determines the structure of proteins

• sequence of triplet bases in the gene for that protein determines order of amino acids

• specific order of amino acids determines the proteins shape

• proteins shape determines function

• proteins form enzymes, structural proteins (collagen) or hormones

Diagram: a cell making an enzyme (protein), shows a small part of one strand of DNA

• the cell reads the DNA sequence as triplets of bases

• each triplet encodes for a specific amino acid

Protein synthesis stage 1

1. Transcription - the base sequence (ATCG) of a gene is copied into a complementary template molecule called messenger RNA (mRNA).

   • mRNA is single-stranded molecule.

   • mRNA now passes out of the nucleus into the cytoplasm.

Protein synthesis stage 2

2. Translation

   • mRNA attaches to a ribosome.

   • Amino acids are brought to the ribosome on carrier molecules called transfer RNA (tRNA)

   • ribosome reads the triplet bases on mRNA

   • uses this to join together the correct amino acids in the correct order

   • once protein chain is complete, it folds into its unique shape which lets the protein do its job

v6 - protein synthesis

Regular mutation in DNA which codes for proteins

• mutation = a change to a base

• happen very often

• different base triplets can sometimes encode for the same amino acid = most mutations have no effect on the proteins shape/function

Bad mutation in DNA which codes for proteins

• sometimes a mutation can lead to the protein having a different amino acid = shape change has a dramatic effect on a proteins function

  • e.g active site of an enzyme may change shape, so it can no longer attach to the substrate

  • if structural protein (collagen) changes shape then it may lose its strength

Mutation in a part of DNA that doesn’t code for proteins

• non-coding parts of DNA switch regions of genes on and off

• they tell genes when to produce proteins

• mutations in these affect how genes are switched on or off

  • e.g a gene turned on when it should be off

  • meaning a cell produces a protein that it’s not meant to have at that time

  • can be a very significant effect e.g uncontrolled mitosis leading to cancer

v7 - Mutations

Who do chromosomes come from

• 1 in the pair is from your father, and 1 your mother

Genes controlling characteristics, amount of copies of a gene

• most characteristics are controlled by many genes acting together (height)

• since chromosomes come in pairs, we have 2 copies of every gene

Alleles

• versions of a gene

Gene for ear wax example

• has 2 alleles - wet (E), dry (e)

• ear wax is controlled by a single gene (2 copies of this)

• wet ear wax is dominant to the allele for dry ear wax

• dry ear wax allele is recessive

• 1st photo = phenotype is wet

• 2nd photo = phenotype is dry

Genotype

• tells us the alleles present

homozygous

• 2 copies of the same allele

phenotype

• tells us characteristics caused by the person’s alleles

heterozygous

• 2 different alleles

Dominant vs recessive

• a dominant allele will show in the phenotype even with 1 copy present

• a recessive allele will show in the phenotype only if 2 copies are present

v8 - alleles

Cystic fibrosis and punnet square

• inherited disorder of cell membranes

• controlled by a single gene

• 2 alleles: C = normal cell membrane function (dominant), c = defective cell membrane (recessive)

• Carrier = Cc OR cC, carrying a single allele but not having the disorder

• Punnet square ratio of carrier to not affected is 1:1

Allele ratio issues

• e.g on average 50% will be carriers and 50% won’t

• just probabilities means it’s possible that all offspring could be carriers OR all could be unaffected

v9 - cystic fibrosis

Polydactyly

• inherited disorder where people have extra fingers or toes

• caused by dominant allele = you can’t be a carrier of it - this is true to any dominant allele because if you have a dominant, you will have the characteristic

• in the e.g the ratio is 1:1

Solution to inherited disorders

Embryo screening:

• embryos are tested to see if they have the alleles for inherited disorders

• embryos that don’t have the defective alleles are implanted into the woman = develop into healthy offspring

Issues of embryo screening

• expensive - some think money should be spent elsewhere in the Health Service

• often a large number of embryos are created but small number are implanted = healthy embryos are destroyed and some think this is unethical

• in the future we may be able to screen embryos to produce offspring with desirable features e.g taller or more intelligent offspring which people think is unethical

Gene therapy

• in the future scientists may be able to correct faulty alleles and use this to treated inherited disorders - still experimental

v10 - polydactyly

Family tree - only shows phenotypes, not genotypes

practice question: work out genotype of person 2

• CF allele is recessive = person 2 must have the genotype cc.

Family tree - only shows phenotypes, not genotypes

practice question: give one piece of evidence that CF is caused by a recessive allele

• person 11 has CF but neither parent has it

• 7&8 are carriers and the CF allele must be recessive

• if CF was dominant, then at least 1 of the parents would have to have CF in order to pass the allele on to person 11

Family tree - only shows phenotypes, not genotypes

practice question: if person 7&8 had another child, what’s the chance that it would have CF?

• 7&8 are both carriers (heterozygous) = cC or Cc

• 1 in 4 chance

Family tree - only shows phenotypes, not genotypes

practice question: what’s the genotype of person 3?

• alleles are inherited each from 1 parent

• polydactyly is dominant

• person 1 must be pp and person 2 can be PP or Pp/pP

• person 3 has p from person 1 and has P from person 2 because they have polydactyly

• person 3 = pP/Pp

Family tree - only shows phenotypes, not genotypes

practice question: if person 6 and 7 have another child, whats the probability that they have poldactyly?

• person 6 = pP/Pp

• person 7 = pp

• probability = 50%

v11 - family trees

Inheritance of sex

• 22 chromosome pairs contain the genes that determine inherited characteristics only

• 1 of the pairs contains genes that determine sex

  • males = XY

  • females = XX

Inheritance of sex by punnet square

• just probabilities - several offspring and ALL could be male or female

v12 - inheritance of sex