Inheritance and genetics

asexual reproduction

• involves one parent

• no fusion of gametes

• offspring are 2 genetically identical daughter cells

• mitosis in eukaryotic cells

• binary fission in bacteria cells

sexual reproduction

• sex cells called gametes are produced

• new cell formed is zygote

• male and female form of gamete

Gametes in humans

• sperm cells are produced in testes

• egg cells produced in ovaries

• cells inside these organs divide by meiosis

• meiosis produce 4 genetically identical offspring

• product of meiosis is used in sexual reproduction

Meosis

• cell doubles chromosomes DNA replication

• chromosomes arrange themselves in pairs at the centre of the cell

• pairs are pulled apart (division 1)

• chromosomes line up at the centre again

• arms of chromosomes are pulled apart (division 2)

all genetically different

Advantages of sexual reproduction

• produces variation in offspring

• variation gives survival advantages by natural selection

• natural selection can be speeded by humans in selective breeding to increases food production

Advantages of asexual reproduction

• only one parent needed

• more time and energy efficient

• faster

• may genetically identical offspring can be produced when conditions are favourable

DNA

• chromosomes are made up of long molecules of deoxyribonucleic acid DNA

• polymer

• double helix

• each chromosome contains thousands of genes made of amino acids

• enzymes control cell chemistry

human genome

• entire genetic material of the organism

• mitochondria have own DNA (inherited from mother)

• make different proteins

Structure of a nucleotide

• made up of base sugar and phosphate

• 2 pairs A & T and C&G

• held together by hydrogen bonds

• shape of double helix

Genes

• chromosomes divided up into sections of DNA

• each gene contains coded genetic information to make one protein

• this genetic code is coded by base pairs

genetic code

• sequence base code for amino acids makes protein

• controls cell activity and structures

• 1 gene code for 1 protein

• polypeptides and dipeptides hold amino acids together

triplet code

• a sequence of 3 base codes for one amino acids

• also known as a codon

How proteins work

•  different amino acid sequences create different proteins

•  When the protein chain is complete it folds up to form a unique shape

•   This unique shape enables the proteins to do their job as enzymes, hormones or forming structures in the body such as collagen.

Protein synthesis

• Transcription

• mRNA can move out of nucleus & attaches to ribosomes

• to correct the sequence of amino acids are brought to ribosomes and joined together (translation)

• this amino acid sequence forms into a protein

transcription

DNA can’t travel out of the nucleus to the ribosomes (too big) so the base code of each gene is transcribed (copied) onto an RNA molecule called messenger RNA

mutation

Random changes of gene can occur by changing the bases

Most are harmless and do not alter the protein, or only alter it slightly so that its appearance or function is not changed.

A few mutations can cause:

1. the protein’s shape to be altered. E.g. a different shaped active site on the enzyme will mean that it cannot bind to its substrate or a structural protein (collagen) can lose its strength

2. Different protein to be created and so intended function is not carried out

non-coding DNA

 Some parts of the DNA are not used for coding

 These parts are used to either switch off or switch on a gene

 If mutations occur here, a gene could be expressed (Switched on) and so make a lot of proteins or be switched off and not make enough proteins

what is an allele

• alternate version of the same gene

• one from mother one from father

• homologous pairs → code for same parts of the body

phenotype

actual characteristic expressed by the person’s gene

genotype

the combination of alleles that a person has that codes for their specific phenotype

homozygous

two of the same alleles

heterozygous

two different alleles

monohybrid cross rules

• always use the same letter for the two alleles but use uppercase for the dominant allele and the lowercase for the recessive

• when yous show breeding use an X

• put phenotypes in circles

• show all possible outcomes

• give answers as a percentage

monohybrid cross

when you look at one characteristic from a man and a woman and see what their offsprings could have

both have two alleles each & can pass either one to their offspring in either their sperm/cell

genetic conditions

• inherited in your genes from your parents

• colour blindness,cystic fibrosis, sickle cell anemia

• polydactyl (having extra fingers) caused by dominant allele

• cystic fibrosis is a recessive disease (f) only inherited if (ff)

For and against embryo screening

For	Against
help people stop suffering	implies genetic disorders are unwanted/undesirable
treating disorders are expensive	slippery slope → people could chose features of child
	expensive

Screening for genetic disorders

two main methods to screen embryos:

amniocentesis

chorionic villus sampling

IVF

amniocentesis

• takes fluid from around developing fetus

• contains fetal cells used for genetic screening

chorionic villus sampling

• takes small tissue from developing placenta

Carrying out embryonic screening

• DNA isolated from the embryo cells and tested for specific disorders

• if screening shows that a fetus is affected the parents have a choice

adv and disadv of embryo screening

adv	disadv
could reduce high societal cost of healthcare and support children with genetic disorders	increase risk of miscarriage
avoid children being born into pain/suffering	can give false positive/negative result

selective breeding

• artificial selection of parents with desired characteristics from a mixed population

• bred together

• offspring with desired characteristics

• are bred together

• continues over many generations until all show desired characteristics

Inbreeding

consequences of selective breeding

gene pool reduced

all animals closely related

diseases could wipe out the whole population as the genes have a similar weakness

genetic engineering

a process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic

Genetic engineering to produce insulin

• bacterium with ring of DNA called a plasmid

• plasmid taken out of bacterium and split open by enzyme

• insulin gene cut out by an of DNA by enzyme

• plasmid with insulin gene in it taken by bacterium

• bacterium multiplies many times

• insulin gene is switched on & insulin is harvested

steps to adult cell cloning

1. nucleus removed from an unfertilised egg cell

2. nucleus taken from adult body cell e.g skin cell

3. nucleus from adult cell is inserted in empty egg cell

4. new egg cell is given a tiny electric shock → stimulates to start dividing to form embryo cells

5. when embryo has developed into ball of cells it’s inserted into a womb of adult female to continue development

benefits and risks of cloning

benefits	risks
produce useful proteins in milk	cloning animals
medically useful	produce lots of organisms with identical genes
save animals from extinction	reduces variety in population
bring back extinct animals	less likely to survive

tissue culture

• plant cells are taken from parent

• grown in growth medium with hormones

• new plants are made - clones of the original parent plant due to meristem tissue throughout the plant

• useful as we can preserve rare species of plants and grow them in plant nurseries to produce more

plant cuttings

• take cuttings from older plants

• plant them with growth hormones to produce clones quickly

• this method is older, cheaper and simpler than tissue culture (doesn’t make as many plants)

embryonic cloning

• splitting apart cells from a developing animal embryo before they became specialised

• then transporting the identical embryo into host mothers