Nat5 Biology - Key Area 2.4 + 2.5

What is variation?

Variation is the differences in characteristics between members of the same species.

How does variation occur?

Combining genes from two different parents contributes to variation within a species.

What is the importance of variation?

Variation within a population makes it possible for a population to evolve over time in response to changing environmental conditions. *

What are the two types of variation?

Discrete and Continuous.

Discrete variation

Discrete variation is caused by single gene inheritance. The characteristics of an organism show discrete variation where measurements fall into distinct groups. E.g. blood groups, tongue rolling ability, fingerprint pattern.

Continuous variation/polygenic inheritance

Continuous variation is caused by multiple genes. This is also called Polygenic inheritance. Polygenic inheritance of characteristics shows continuous variation where there is a range of values between a minimum and a maximum. E.g. height, hand span, foot length.

Allele

Different forms of a gene that produces different phenotypes.

Gene

A sequence of DNA on a chromosome that contains coded instructions for making a specific protein.

Dominant

An allele that always produces a certain phenotype in an organism. It is represented by a capital letter e.g. R, T, S.

Recessive

An allele that is not shown in the phenotype if a dominant allele of the same gene is present. Represented by a lower-case letter e.g. r, t, s.

Homozygous

An individual who has inherited the two of the same alleles for a gene e.g. RR or rr.

Heterozygous

An individual who has inherited two different alleles of the same gene e.g. Rr.

Genotype

An individual's alleles for a particular characteristic.

Phenotype

Physical appearance based on the genotype for a particular characteristic.

P

Parents.

F1 generation

Children.

F2 generation

Grandchildren.

What is a monohybrid cross?

A monohybrid cross is a genetic cross which occurs between two organisms of the same species and focuses on the chance (%) of a single characteristic being inherited by the offspring following sexual reproduction.

If both parents are homozygous (true breeding) for different alleles:

all of the F1 generation will be Bb.

When the F1 generation are self-fertilised:

there will be a 3: 1 ratio of dominant individuals to recessive.

Upper epidermis

Transparent layer of cells which is protective and allows light to pass to the cells containing chloroplasts.

Mesophyll cells

Cells contain many chloroplasts for photosynthesis .

Vein

Made up of xylem and phloem to transport substances.

Guard cells

Surrounding stomata and assist in opening and closing of the stoma. Contain chloroplasts for photosynthesis.

Stoma(ta)

Pores surrounded by guard cells on lower epidermis that allow gas exchange and transpiration.

What does the stem of the plant consist of?

Xylem and phloem.

Xylem

transports water + minerals upwards, non-living, made up of rings of lignin to withstand pressure changes as water moves through plant, it is hollow.

Phloem

transports sugar upwards + downwards, living, contains sieve plates, has associated companion cells.

Transpiration

Transpiration is the process of water moving up through a plant and its evaporation through the stomata.

Transpiration: root hair cell (step 1)

The process of transpiration begins with osmosis. Water moves from a high water concentration in the soil to a low water concentration in the root hair cell. (Root hair cells provide a large surface area for the absorption of water by osmosis.

Transpiration: root hair cell (step 2)

Water enters the root hair cell and is transported through the root cortex cell until it reaches the xylem vessel, which carries the water upwards towards the leaf.

Transpiration: root hair cell (step 3)

Water is pulled up from the xylem vessels to the leaves where water molecules exit and move from cell to cell until they reach the mesophyll cells (main site of photosynthesis). Some of the water evaporates into surrounding air.

Factors affecting transpiration

Temperature, wind speed, surface area, and humidity. Increasing temperature, wind speed and surface area will increase the rate of transpiration. Increasing humidity will decrease the rate of transpiration.

Bubble potometer

A bubble potometer can be used to measure the volume of water absorbed by a leafy shoot. The faster the bubble moves, the greater the rate of water uptake and so the greater the rate of transpiration.

Mass potometer

A mass potometer measures the change in mass of a plant as a result of evaporation of water (transpiration). As water evaporates through the leaves and the stem, the decrease in mass will be recorded as a measurement of transpiration.