P = chromosomes condense, membrane dissolves and crossing over happens with paired chromosomes (bivalents) M= spindle fibres connect to bivalents to align them in the middle A = spindle fibres contract and homologous chromosomes are pulled away to opposite ends T = chromosomes decondense and cell divides to 2 haploid cells through cytokinesis

Describe the stages of meiosis II?

P = chromosomes condense, membrane dissolves M = spindle fibres attach to chromosomes and aligns them in the middle A = spindle fibres pulls sister chromatids away seperating them T = chromosomes decondense, membrane reforms and cell divides to form 4 haploid cells

Why can the 4 haploid cells from meiosis be genetically distinct?

Because of crossing over in prophase 1 can cause recombination

Explain how crossing over of chromsomes work and when does it happen?

Prophase 1 in meiosis homologous chromosomes are paired up to form bivalents held together at chiasmata crossing over happens at chiasmata which can form new combos forms recombinant chromatids

What is the process called whereby chromosomes forms pairs? what are these pairs called?

Synapsis forms bivalents

How are the pairs of chromosomes held together in prophase 1?

By the chiasmata

How and where does crossing over occur?

In prophase 1 in meiosis crossing over occurs at the chiasmata which forms recombinant chromatids by exchanging genetic material

How does metaphase 1 lead to more variation?

Due to random independent assortment of chromosomes into the gametes when aligning in the middle

What would happen if the chromosome numbers were not halved?

The total chromosome numbers would double in each generation = polyploidy

What are the 3 ways genetic variation is achieved?

Crossing over in prophase 1 random assortment in metaphase 1 random fusion of gametes

What is non-disjunction? (2)

When chromosomes don't separate properly = extra or missing chromosomes aneuploidy

When can non-disjunction happen? (2)

When chromosomes don't separate in anaphase 1 = affects all 4 when chromosomes dont separate in anaphase 2 = affects 2

How is down syndrome caused?

3 copies of chromosome 21 ( trisomy)

What can influence the chances of non-disjunction occuring?

Maternal age because of developing oocytes being in p1 until ovulation especially after 30 mostly from non disjunction in meiosis 1

What is karyotyping? how is it done?

Chromosomes are organised to find genetic abnormalities cells are harvested from fetus before cell division so that the chromosomes are visible

What is chorionic villi sampling?

Removes placental tissue at 11 weeks to find CS abnormalities 1% risk of miscarriage

What is amniocentesis

Amniotic fluid at 16 weeks to find CS abnormalities with 0.5% chance of miscarriage

What are the differences between mitosis and meiosis? (8)

Division – Mit 1 division mei 2 divisions Independent assortment – Homologous pairs are randomly separated into separate cells only in meiosis Synapsis – Homologous pairs form bivalents in meiosis only Crossing over – Non-sister chromatids of homologous pairs may exchange genetic material in meiosis only Outcome – Mitosis forms 2 cells meiosis forms 4 Ploidy – mitosis forms diploid while meiosis forms haploid Use – Mitosis is used to clone body cells, while meiosis is used to generate sex cells (gametes) Genetics – Cells produced by mitosis are genetically identical (clones), while cells produced by meiosis are genetically distinct

What are the 3 factors that cause mutations?

Radiation - UV from sun, gamma, X-rays chemical: reactive oxygen species (pollutants) biological: bacteria and viruses

What are Mendel's laws? (3)

Law of segregation: alleles separate when gametes form law of independence: alleles separate independently principle of dominance: dominant alleles mask recessive alleles

How did Mendel discover these laws? (3)

Large number of pea plants were crossed 1) crossed a variety of purebred peas 2) crossed the offspring with each other 3) crosses performed again and again for reliable results

How did the results show these laws?

The offspring only expressed 1 allele not a mix (shows dominance)

What was the normal ratio expressed in mendel's law?

3:1

What are the exceptions of mendel's laws? (2)

Linked genes: genes located on the same chromosome some genes show co-dominance or incomplete dominance

What are gametes? how are they made haploid? (2)

Haploid cells formed by meiosis separated into different nuclei in meiosis 1 separating the allele pairs

What happens to number of alleles when gametes fuse?

Zygote will have 2 alleles for each gene

What is an exception to having 2 pairs of each gene?

Males sex chromosome only have 1 allele for each gene because XY arent paired

What does homozygous mean in terms of genes?

Maternal and paternal alleles are the same

What does heterozygous mean in terms of genes?

Maternal and paternal alleles are different

What does hemizygous mean? and what is considered to be hemizygous?

Male sex chromosomes because only 1 allele for each gene is located on the chromosome

What is genotype? (2)

Gene composition for a specific trait can be hetero or homozygous

What is phenotype?

Observable characteristic of a specific trait determined by genotype and environment

What is codominance

When both alleles are expressed equally Eg: black and white feathers Csmall B CsmallW CBCW

Explain the dominance of ABO blood group

A and B are co-dominant and change the antigen structure O is recessive and doesnt change the antigen structure blood group alleles are represented as I smallA IsmallB or i

How are genetic diseases caused?

Mutations to genes affect cell function

What is an example of autosomal recessive disease?

Cystic fibrosis

What is an example of autosomal dominant disease?

Huntingtons disease

What is an example of codominance disease?

Sickle cell anaemia

What is cystic fibrosis? (5)

Autosomal recessive disorder mutation to CFTR gene on chromosome 7 produce thick sticky mucus which clogs airways and digestive ducts respiratory failure and pancreatic cysts heterozygous people will only be a carrier