Mendelian Genetics

purpose of meiosis

reduces the chromosome number in half

key features of meiosis

one round of DNA replication → 4 genetically unique daughter cells

sister chromatids - identical copies of a chromosome

homologous chromosomes - contain genes for same traits but aren’t identical

Meiosis I

seperates homologous chromosomes

synapsis

chiasmata

synapsis

homologous chromosomes pair up

held together by synaptonemal complex

chiasmata

the crossing-over points where non-sister chromatids exchange genetic material

Meiosis II

seperates sister chromatids into individual chromosomes

NO chromosome replication occurs between meiosis I and II

end result is 4 HAPLOID gametes and a halving of chromosome number

when does DNA replication occur in mitosis

during interphase before nuclear division begins

when does DNA replication happen in meiosis

occurs once during interphase before meiosis I begins

function of mitosis

allows multicellular adult to arise from a zygote

produces cells for growth and repair

function of meiosis

produces gametes

reduces chromosome number by half

genetic variation amongst gametes

independant assortment of chromosomes

each chromosome pair sorts independantly

umans have 2²³ (~8.4M) possible chromosome combinations

crossing over

occurs in prophase I

produces recombinant chromosomes wit new allele combinations

humans average 2-3 crossover events per chromosome pair

random fertilization

any sperm can fuse with any egg

~70 trillion possible zygote combinations (not incl crossing over)

why is sex important

genetic variation allows populations to adapt

natural selection favours beneficial combinations

heritable variation enables evolution

comparative genetics

studies how genes and genomes vary across species

helps understand :

human evolution

genetic diseases

lifespan variation (eg bats have long lifespans despite high metabolic rates)

character

a heritable feature that varies among individuals

eg eye colour

trait

a specific variant of a character

eg brown eye

true breeding

offspring always have the same traits

p generation

true breeding parental generation

hybridization

crossbreeding two different true-breeding varieties

f1 generation

first generation offspring of hybridisation

F2 generation

generation from allowing F1 hybrids to self-pollinate

alleles

alternate versions of genes

monohybrids

F1 generation hybrids from breeding experiments following a single character

eg flower colour

dihybrids

F1 generation hybrids from breeding whic crossed two characters

eg flower colour and seed shape

law of segragation

each organism inherits two alleles (one from each parent)

alleles seperate during gamete formation (each gamete gets one allele)

law of independant assortment

genes for different traits segregate independantly

only applies to genes on different chromosomes or far apart on the same chromosome

complete dominance

dominant allele masks recessive allele

Purple flower (dominant) x White flower (recessive) → all purple F1

incomplete dominance

F1 hybrids have a mix of both traits

eg red x white snapdragon → pink flowers

codominance

both alleles affect phenotype equally

eg AB blood type (both A & B antigens are expressed)

multiple alleles

genees with more than two alleles

eg ABO blood types

pleiotropy

one gene affects multiple traits

eg sickle-cell disease (affects blood, organs, resistance to malaria)

epistatis

one gene masks/modifies another genes’s effect

eg coat colour in mice (one gene controls pigment, another controls pigment deposition)

polygenic inheritance

multiple genes control a single trait

eg human skin colour (influenced by many genes, leading to a continuous gradient)

phenotype

genetics & environment

eg human heigh is affected by nutrition

multifactorial characters

several genes and different environmental influences impact the same caracter

eg skin colour in humans

pedigree analysis

used to track inherited traits over generations

can predict genetic disorders

recessive disorders

cystic fibrosis - defective chloride transport

tay sachs disease - defective enzyme → lipid buildup in brain

sickel cell disease - abnormal haemoglobin

dominant disorders

achondroplasia (dwarfism)

huntington’s disease - nervous system degeneration

genetic testing & fetal screening

harmony test (cfDNA) - non invasive prenatal genetic screening

inbreeding risks - increases genetic disorders due to recessive alleles