inherence variation and evolution

What is sexual reproduction?

● Type of reproduction.

● Involves the production of gametes by meiosis.

● A gamete from each parent fuses to form a zygote.

● Genetic information from each gamete is mixed so the resulting zygote is unique.

What are gametes?

sex cells, haploid

what is meiosis?

• Form of cell division involved in the formation of

gametes (non-identical haploid cells) in reproductive

organs.

. Chromosome number is halved.

• Involves two divisions

what must occur prior to meiosis?

Interphase - copies of genetic information are made during this process.

what happens during the first stage of meiosis?

● Chromosome pairs line up along the cell equator

● The pair of chromosomes are separated and move to opposite poles of the cell (the side to which each chromosome is pulled is random, creating variation)

● Chromosome number is halved

what happens during the second stage of meiosis?

● Chromosomes line up along the cell equator.

● The chromatids are separated and move to opposite

poles of the cell.

● Four unique haploid gametes are produced.

what is meiosis important for sexual reproduction?

• It increases genetic variation.

• It ensures that the zygote formed at

fertilisation is diploid

describe fertilisation and its resulting outcome

Gametes join together to restore the normal number of chromosomes and the new cell then divides by mitosis (which increases the number of cells). As the embryo develops, cells differentiate.

describe the circumstances in which malarial parasites reproduce sexually and asexually

sexual reproduction in the mosquito

asexual reproduction in the human host

describe the circumstances in which fungi reproduce sexually and asexually

Asexual reproduction by spores.

Sexual reproduction to give variation.

describe the circumstances in which plants reproduce sexually and asexually

sexual reproduction to produce seeds.

asexual reproduction by runners or bulbs division

What is DNA?

A double-stranded polymer of nucleotides, wound to form a double helix

the genetic material of the cell found in its nucleus

define genome

The entire genetic material of an organism

what is understanding the human genome important?

Why is understanding the human genome

important?

PDF

https://www.physicsandmathstutor.com/biology-revision/gcse-aqa/inheritance-variation-evolution/rep

The whole human genome has been studied and is

important for the development of medicine in the future.

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• Searching for genes linked to different types of disease.

• Understanding and treating inherited disorders.

• Tracing human migration patterns from the past.

What is a chromosome?

a long continuous thread of DNA that consists of numerous genes

how many chromosomes do human body cells have?

46 chromosomes (23 pairs)

how many chromosomes do human gametes have?

23 chromosomes

Define gene

A small section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein.

What are alleles?

Different forms of the same gene

What is a dominant allele?

A version of a gene where only one copy is needed for it to be expressed

What is a recessive allele?

A version of a gene where two copies are needed for it to be expressed

what is meant when an organism is homozygous?

When an organism has two copies of the same allele (two recessive or two dominant).

what us meant when an organism is heterozygous?

when an organism had two different versions of the same gene (one dominant and one recessive)

What is a genotype?

the genes present for a trait

What is a phenotype?

the visible characteristic

How are dominant alleles represented in a punnett square?

They are represented using uppercase letters.

how are recessive alleles represented in a punnett square?

They use the lowercase version of the same letter as the dominant allele.

what is the problem with single gene crosses?

Most characteristics are controlled by multiple alleles rather than just one

what is an inherited disorder

A disorder caused by the inheritance of certain alleles.

give 2 examples of inherited disorders

● Polydactyly (having extra fingers or toes) - caused by a dominant allele.

● Cystic fibrosis (a disorder of cell membranes) - caused by a recessive allele.

how are embryos screened for inherited disorders?

During IVF, one cell is removed (from an 8 cell embryo) and tested for disorder-causing alleles. If the cell doesn't have any indicator alleles, then the originating embryo is implanted into the uterus.

what are the ethical issues concerning embryo screening?

● It could lead to beliefs in society that being disabled or having a disorder is less human or associated with inferiority.

● The destruction of embryos with inherited disorders is seen by some as murder as these would go on to become human beings.

● It could be viewed as part of the concept of designer babies as it may be for the parents convenience or wishes rather than the child's wellbeing.

what are the economic issues concerning embryo screening

● Costs of hospital treatment and medication will need to be considered if it is known that a child will have an inherited disorder and financial support explored if necessary.

what are the social issues concerning embryo screening?

● Social care for children with inherited disorders may

need to be considered if parents are unable to provide

care.

● If an embryo is found to have an inherited disorder and

is terminated, this can prevent a child and its parents

from potential suffering in the future due to the disorder.

What is gene therapy?

The insertion of a normal allele into the cells of a person with an inherited disorder to functionally replace the faulty allele.

what are the ethical issues concerning gene therapy?

● Some people believe that it is going against and 'playing God'.

● The introduced genes could enter sex cells and so be passed to future generations.

what are sex chromosomes?

A pair of chromosomes that determine sex:

● Males have an X and a Y chromosome

● Females have two X chromosomes

why does the inheritance of a Y chromosome mean that an embryo develops into a male?

testes development in an embryo ks stimulated by a gene present on the Y chromosome

What is a sex-linked characteristic?

A characteristic that is coded for by an allele found on a sex chromosome.

why are the majority of gene found on the X chromosome rather than the Y chromosome?

the X chromosome is bigger than a Y chromosome so more genea are carried on it

why are men more likely to show the phenotype for a recessive sex-linked trait than women?

-Many genes are found on the X chromosome that have no counterpart on the Y chromosome.

-Women (XX) have two alleles for each sex-linked gene whereas men

(XY) often only have one allele.. only one recessive allele is required

to produce the recessive phenotype in males.

What is variation?

Differences in the characteristics of individuals in a population

what are the 2 causes of variation within a species?

-genetics

-environment

-a mixture of both of the above

what is genetic variation?

● Variations in the genotypes of organisms of the same species due to the presence of different alleles

● Creates differences in phenotypes

what creates

-spontaneous mutations

-sexual reproduction

What is a mutation?

a random change to the base sequence in DNA which results in genetic variants. they occur continuously.

state the 3 types of gene mutations

-insertion

-deletion

-substitution

How may a gene mutation affect an organism's phenotype? (3)

● Neutral mutation does not change the sequence of amino acids. Protein structure and function same. No effect on phenotype.

● Mutation may cause a minor change in an organism's phenotype e.g. change in eye colour.

● Mutation may completely change the sequence of amino acids. This may result in a non-functional protein. Severe changes to phenotype.

what is the consequence of a new phenotype caused by a mutation being suited to an environmental change?

there will be a rapid change in the species