AoS 1: how is inheritance explained?

**DP1: the distinction between genes, alleles and a genome**

somatic cells

any cell other than a gamete cell.

chromosomes

a thread-like structure containing DNA coiled around histones

DNA

contains genetic instructions.

amount of DNA in a human cell

2 metres

genes

a section of chromosomes that code for a gene product through the order of the nucleotide base sequences it posseses.

eukaryotic chromosomes structure

consist of DNA and histones

* coiling allows DNA to be condensed into chromosomes

histones

a protein found in eukaryotic organisms that assist in DNA packaging

nucleosome

a section of DNA coiled around a histone

DNA in prokaryotic cells

one large circular chromosome + several small chromosomal structures called plasmids

* located in cytosol
* usually a singe circular chromosome
* no histones, not as condensed

DNA structure

* double helix
* anti-parallel
* the two strands run in opposites directions

nucleuotide

a sub-unit of DNA consisting of a phosphate group, a deoxyribose sugar and a nitrogenous base.

* assembled head to tail, forming a chain

the 4 nitrogenous bases

* adenine
* thymine
* cytosine
* guanine

adenine

one of the purine bases found in nucleotides

thymine

one of the pyrimidine bases found in nucleotides

guanine

one of the purine bases found in nucleotides

cytosine

one of the pyrimidine bases found in nucleotides

chagraff’s rule

adenine bonds with thymine and cytosine bonds with guanine through hydrogen bonds.

* additionally, it states the relative proportions of bases A and T are equal and C and G are equal.

gene products (proteins)

control a particular characteristic or trait

alleles

alternative forms of a gene

genome

the complete set of genetic instructions for an organism; the total DNA of an organism

genomics

the field of study of genomes

zygote chromosome formation

consists of 46 chromosomes, one from each parent

* therefore diploid

diploid

having two copies of each specific chromosome

genome determination

measure the haploid number of base pairs in a cell

haploid

having one copy of each specific chromosome

human genome

3 234 830 000 base pairs

nuclear DNA

all the DNA in the nucleus of the cell.

**DP2: the nature of a pair of homologous chromosomes carrying the same gene loci and the distinction between autosomes and sex chromosomes**

homologous

matching pairs of chromosomes that have the same genes at the same positions or loci

gene locus

the position of a gene on a chromosome

* loci = plural

non-homologous

chromosomes that do not have the same gene loci

ideogram

a stylised representation of a haploid set of chromosomes arranged by decreasing size

homologous chromosome similarities

* length/size
* centromere position
* gene loci
* banding pattern

maternal homologue

homologous chromosome from the mother

paternal homologue

homologous chromosome from the father

two types of chromosomes

* autosomes
* allosomes/sex chromosomes

allosomes

involved in sex determination

* 23rd pair of chromosomes
* male: XY
* female: XX

autosomes

any chromosomes that aren’t involved in sex determination

* first 22 pairs of chromosomes
* distinguished by their size, centromere position and pattern of banding

centromere

the position where the chromosomes are held together

amount of genes on sex chromosomes

X chromosome: 800

Y chromosome: 50

sex determination

SRY gene on the Y chromosome codes for male characteristics, Y chromosome determines gender

environmental sex determination

the sex of the offspring is determined by environmental conditions rather than genetic factors

**DP3: variability of chromosomes in terms of size and number in different organisms**

sister chromatids

identical copies of DNA formed by the replication of a chromosome

chromatid arms

* smaller arm = p arm


* bigger arms = q arms

centromere classification

* metacentric
* submetacentric
* acrocentric
* telocentric

metacentric

centromere is located at the middle of the chromosome

submetacentric

chromosome is located towards one end of the chromosome

acrocentric

centromere is located very close to one end of the chromosome

telocentric

chromosome is located at the tip of the chromosome

**DP4: karyotypes as a visual representation that can be used to identify chromosome abnormalities**

karyotype

an image of chromosomes from a cell organised from size, centromere position and banding patterns

* organised by international convention

cytogeniticists

scientists who specialise in the study of the human karyotype

trisomy

a condition in which a cell or organism has three copies of a particular chromosome that is normally a homologous pair.

monosomy

a condition in which a cell or organism has one copy of a particular chromosome that is normally a homologous pair.

* results in embryotic death unless sex cell effected

trisomy example

down syndrome, klinefelter syndrome

monosomy example

turners syndrome

errors in chromosomes cause

non-disjunction in anaphase of meiosis/mitosis resulting in an imbalance of chromosomes

duplication of a chromosome

type of chromosome change in which part of a chromosome is repeated

deletion of a chromosome

type of chromosome change in which part of a chromosome is lost

translocation of a chromosome

where the location of a chromosome segment is altered so it relocated to a new region

**DP5: the production of haploid gametes from diploid cells by meiosis, including the significance of crossing over of chromatids and independent assortment for genetic diversity**

reduction division

production of gametes through cellular division in meiosis I then reduces the chromosome number from diploid to haploid

advantages of sexual reproduction

offspring differ genetically from one another and their parents allowing species to adapt to different environments

prophase I

* chromosomes condense
* nuclear envelope breaks down
* crossing over occurs

metaphase I

* homologous chromosomes line up along the equator

anaphase I

* spindle fibres pull each chromosome to the pole, separating them

telophase I

* nuclear envelope may form again


* cytokinesis occurs

cytokinesis

the process of a cell dividing into two daughter cells

interphase

DNA is replicated during ‘S phase’ producing sister chromatids

prophase II

* chromosomes condense
* nuclear envelope breaks down

metaphase II

* chromosomes line-up along the equator of the cell

anaphase II

* replicated chromosomes are separated into chromatids by spindle fibres

telophase II

* a new nuclear envelope forms


* cytokinesis occurs, creating 4 haploid daughter cells

methods for genetic variation

* crossing over
* independent assortment

crossing over

an event that occurs during meiosis involving the exchange of corresponding segments of non-sister chromatids of homologous chromosomes

* become connected at the chiasma, becoming bivalent

bivalent

a pair of homologous chromosomes that are held together by a crossover

crossing over process

when connected, the chromatids become entangled, DNA breaks and recombines crossing new combinations of alleles

independent assortment

the formation of random chromosome combinations during meiosis

independent assortment process

during meiosis, the chromosomes line up in the middle of the cell randomly producing different possibilities of how chromosomes can be orientated towards each other

**DP6: the use of symbols in the writing of genotypes for the alleles present at a particular gene locus**

genotype

the underlying genetical make-up that determines an organisms phenotype

* not visible

genotype determination

determined by combination of alleles

homozygous

having two identical alleles for a gene

* can be either homozygous dominant or recessive

heterozygous

having two different alleles for a gene

hemizygous

the genotype on either the X or Y chromosome -- which comprises of just a singular allele for a gene

genotype for genes on sex chromosomes

females may either be homozygous or heterozygous, whereas, males are hemizygous

* males are more susceptible to X-linked traits.

writing a genotype with two expressions

use symbols:

* dominant traits are represented by a capital letter (W)
* recessive traits are represented by lowercase letters (w)

writing a genotype with multiple expressions

common letter is used with superscripts added

writing a genotype on the x chromosome

the chromosome must be specified, with the allele represented in superscript

**DP7: the expression of dominant and recessive phenotypes, including codominance and incomplete dominance**

phenotype

the observable or measurable characteristics of an organism

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complete dominance

only one allele is required to express the traits, recessive traits require two copies of an allele to present

carrier

a heterozygote that has the allele for a recessive trait but doesn’t show the trait in their phenotype

* recessive trait can be passed to offspring

codominance

when the heterozygote expresses the dominant and recessive trait of a gene in its phenotype

* type of partial dominance

incomplete dominance

occurs in heterozygotes where the phenotype is an intermediate of traits

**DP8: proportionate influences of genetic material, and environmental and epigenetic factors, on phenotypes**

phenotype factors

genotype + environment

epigenetics

the study of changes in organisms caused by modification of gene expression rather than alteration of genetic code itself.