B6 - Inheritance and Variation

what is a chromosome

a chromosome is a long DNA molecule with part of the genetic material of an organism

where in a cell are chromosomes located?

the nucleus

how many chromosomes are there in a typical human cell?

43

26 pairs

what is the structure of DNA

DNA is a polymer made up of two strands wrapped around each other to form a double helix

what are sex chromosomes?

the 23rd pair of chromosomes

they determine whether an individual is a male or female

what sex chromosomes do females have?

xx

what sex chromosomes do males have?

xy

what is a gene?

a small section of DNA on a chromosome

each gene codes for a particular sequence of amino acids, to make a specific protein

what is the genome of an organism?

the entire genetic material of an organism

how does understanding the whole human genome help science?

1. it could help us understand and treat inherited disorders
2. it could be used in tracing human migration patterns from the past
3. it will allow scientists to search for genes linked to different diseases

what is an allele?

different versions of a particular gene

how many alleles of each gene does an individual have?

2

what does it mean to be homozygous for a particular gene

both alleles of that gene are identical

what does it mean to be heterozygous for a particular gene?

the two alleles of that gene are different

what is a dominant allele?

the one which is always expressed, even if there is one copy of that allele

what is a recessive allele?

the one which is only expressed if there are two copies of it

what does ‘genotype’ mean?

there are two definitions


1. an individual’s collection of genes
2. the two alleles present for a particular gene

\
in either case, the term genotype refers to the specific genes or alleles that an individual has

what does phenotype mean?

the characteristic an organism has as a result of their genotype (and environment)

how are most characteristics determined?

most characteristics are determined by the combination of multiple genes and the environment

what is sexual reproduction

a mode of reproduction in which new offspring are produced by the fusion of male and female gametes

during sexual reproduction in animals, which two gametes fuse together?

sperm cell and egg cell

during sexual reproduction in plants, which two gametes fuse together?

pollen and egg cell

does sexual reproduction lead to genetically identified offspring or genetically unique offspring?

genetically unique offspring

is a sperm cell haploid or diploid?

haploid because it only half the number of chromosomes = 23

what is fertilisation?

the fusion of male and female gametes

what is asexual reproduction?

a type of reproduction where new offspring are produced by a single parent

does asexual reproduction lead to genetically identified offspring or genetically unique offspring?

genetically identical offspring

what is the most common way of reproduction for


1. animals
2. fungi
3. plants
4. bacteria

1. sexual
2. asexual
3. most
4. asexual

what do we call the process by which bacteria reproduce asexually?

binary fission

does asexual reproduction in plants use mitosis or meiosis?

mitosis

what is a gamete and how many chromosomes do they have?

a sex cell

23

where does meiosis take place?

in the sex organs

the testes in males

the ovaries in females

how many cells does meiosis produce and are they genetically identical or unique?

4

genetically unique

when writing genotypes, such as AA, what does the upper-case letter mean?

it means that that allele is dominant

when writing genotypes, such as Aa, what does the lower-case letter mean?

it means that that allele is recessive

what does it mean for an individual to be homozygous dominant or recessive for a gene?

dominant → 2 dominant alleles for that gene e.g. AA HH

recessive → 2 recessive alleles for that gene e.g. aa hh

what is a family tree?

a diagram representing family relationships across multiple generations

also called a pedigree chart

wat is an inherited disease?

a condition that can be passed on genetically through the inheritance of certain alleles

what is cystic fibrosis?

an inherited disorder of cell membranes

it is a recessive condition

it causes sticky mucus to build up in the lungs and the digestive system

what does it mean for an individual to be a carrier of cystic fibrosis?

they are heterozygous and only have one cystic fibrosis allele

this means they can pass it onto their offspring, but do not have the condition themselves

what is polydactyly?

an inherited disorder in which an individual can be born w extra fingers or toes

it is a dominant condition

what is IVF?

the process by which eggs are fertilised by sperm outside the body

the embryos are then grown in a lab until they are ready to be implanted into the mother’s womb

what is embryo screening?

the process of taking a cell from an embryo and analysing its DNA for genetic abnormalities

it’s normally done during the IVF process, before the embryo is implanted into the mother’s womb

how is embryonic screening carried out?

1. DNA is isolated from the embryo
2. probe is mixed with the DNA
3. probe binds with DNA
4. UV light shows the alleles for disorders

give 2 advantages of embryo screening

1. reduces suffering by stopping children from being born with genetic disorders
2. it can save the country a lot of money as treatment for inherited disorders is very expensive

\
this is not an advantage but there are laws in place to stop embryo screening from being abused

give 3 disadvantages of embryo screening

1. screening against inherited disorders may imply that people with such disorders are undesirable - which is not fair or true
2. embryo screening is very expensive process and therefore is not available to all potential parents
3. it could be a ‘slippery slope’ - people may begin screening for less and less harmful conditions, until they start screening for things like height, gender, eye colour etc.

what is variation?

the differences in the characteristics of individuals within a population

which two factors contribute to the variation between organisms?

1. the genes each individual inherits
2. the environments each individual is exposed to

what is natural selection?

the idea that in each generation, the individuals with the most favourable characteristics are most likely to survive and reproduce, therefore passing on their genes

it is the idea that ‘nature’ is ‘selecting’ the fittest individuals

what is evolution?

the process by which the inherited characteristics of a population change over time due to natural selection

in some cases, this may result in the formation of a new species

what does survival of the fittest mean?

the individuals with the most favourable characteristics are most likely to survive

what is speciation?

the evolutionary process by which populations evolve to become distinct species

what is selective breeding?

also known as artificial selection

is the process by which humans use animal or plant breeding to selectively develop characteristics by choosing which males and females will sexually reproduce and have offspring together

describe how the process of selective breeding could be used to increase the yield of tomato plants

1. select the two tomato plants with the highest yield from the population
2. breed them together
3. once the plants have grown and produced tomatoes, repeat the process by breeding together the ones with the highest yield
4. keep repeating this over multiple generations, and the average yield will increase

give examples of traits that might be selectively bred for in plants or animals

plants:

colour of flowers

drought resistance

improved yield of fruits/grains and vegetables

\
animals:

improved yield of meat or milk

desirable size, fur colour, level of aggression

what effect does selective breeding usually have on variation within a population?

it decreases it

what is inbreeding?

the process of mating together closely related individuals

give 3 drawbacks of inbreeding

1. reduces the size of the gene pool
2. rare inherited diseases are more likely to be expressed
3. the population as a whole will be more at risk of disease

what is genetic engineering/modification?

the process of modifying the genome of an organism by introducing a gene from another organism, in order to give a desired characteristic

what do we call crops that have been genetically engineered?

GM crops

how has genetic engineering played a role in the treatment of diabetes

bacterial cells have been genetically engineered to produce human insulin to treat diabetes

give 3 traits that could be altered by genetic engineering in crops?

increased yield

increased quality

resistance to disease

resistance to insects

resistance to herbicides

resistance to droughts

give 2 drawbacks of genetic engineering

1. if the crops breed with wild plants, they could spread the new genes into the wild and affect the wild population’s gene pool
2. there is a small risk that genetically modified food could negatively affect our health

describe the steps of genetic engineering (GM)

1. isolate the desired gene from human DNA
2. insert the DNA into a vector, such as a plasmid
3. insert the plasmid into a host bacterium
4. let the bacterium divide and then produce the desired protein

what is gene therapy?

the treatment of an inherited disorder by giving the patient a healthy copy of the faulty gene

what is a fossil?

fossils are the remains of organisms from millions and billions of years ago, which are found in rocks

why are fossils used as evidence for evolution?

they show how species have changed over millions of years

why are there not many fossils from the earlier organisms on earth?

1. early organisms were soft-bodied, so they did not fossilise well
2. geological activity has destroyed most fossils that formed so long ago

what are the 3 ways fossils formed?

1. hard body parts, such as bones and shells, which decay very slowly, are gradually replaced by minerals, forming rock-like substances
2. animals can leave impressions or casts, such as footprints or burrows, these became covered by layers of sediment, which eventually become rock
3. some parts of organisms may not decay at all, e.g. dead animals and plants could be preserved in amber, peat bogs, tar pits or ice

what does extinction mean?

when there are no remaining individuals of a species still alive

give ways a species could go extinct

1. rapid environmental change such as an increase in temperature
2. reduction in habitat, as it could reduce food and living space
3. high levels of hunting as individuals are directly killed
4. the spread of a new disease
5. competition
6. catastrophic events

what is antibiotic resistance?

the ability of bacteria to survive exposure to antibiotics

how does antibiotic resistance arise?

1. a population of bacteria infect somebody, and that person takes antibiotics to kill bacteria
2. some bacteria have a random mutation that makes them less susceptible to the antibiotic than others (or completely resistant)
3. the antibiotics kill all the bacteria except those that are resistant
4. the resistant strains of bacteria survive and now have more resources to grow and multiple
5. more mutations may arise, leading to greater resistance → there is now a strain of bacteria that can’t be killed by antibiotics, which we call ‘antibiotic-resistant bacteria’

give 3 reasons for the recent rise and spread of antibiotic resistance

1. some doctors prescribe antibiotics inappropriately, such as for non-serious or viral infections
2. some patients don’t complete their course of antibiotics so some bacteria survive, mutate, and develop antibiotic resistance
3. the agriculture industry give huge amounts of antibiotics to farm animals to help them grow more quickly

why is the spread of antibiotic resistance a problem?

if more bacteria are resistant to antibiotics, we won’t be able to treat those infections

this means more people will die of bacterial infections

who was Carl Linnaeus?

18th-century scientist who came up with the binomial system for naming organisms and the Linnean classification system for classifying living organisms

what is the binomial naming system?

every species has a binomial name, which is made up of two parts: the ^^genus^^ name and the ==species== name

e.g. ^^*Homo* ^^==*sapiens*==

\
rules:

both names should be written in italics

all letters should be lowercase except for the first letter of the genus name

give 3 advantages of using a binomial naming system?

1. each species has a unique name → avoids confusion
2. allows scientists from all over the world to discuss species
3. shows how closely related organisms are because closely related organisms often have the same genus name

what is the Linnaeus classification system?

kingdom

phylum

class

order

family

genus

species

who was Carl Woese?

20th-century scientist who used microscopes and modern technology to develop the three-domain system to classify organisms

what are the three domains?

1. Archaea
2. Bacteria
3. Eukaryota → animals, plants, fungi, protists