Genetics Year 10

Genetics

The study of heredity

Heredity

The study of biological inheritance

DNA name

Deoxyribonucleic acid

What is DNA?

A double helix strand of genetic information found in the chromosomes of nearly all organisms. Contains genes that code for different characteristics.

Genes

A segment of DNA made of amino acids; codes for different genotypes and phenotypes

Amino acids

Building blocks of proteins. One amino acid is coded by 3 nitrogenous bases which makes one codon, e.g. AAC, or GCT.

Nitrogenous base complementary pairings

Adenine with Thymine

Guanine with Cytosine

One nucleotide is made of

A phosphate group, pentose sugar and nitrogenous base.

How many chromosomes in a human body?

46 (23 pairs). 22 pairs are autosomes, 1 pair are the sex chromosomes

DNA anti-parallel structure

One side goes up, the other side goes down

Sister chromatids

Replicated forms of a chromosome joined together by the centromere and eventually separated during mitosis or meiosis II.

Diploid cell

A cell containing 2 of each chromosome, one from the mum and one from the dad

Haploid cell

A cell containing one one of each chromosome (only in gametes)

Chromosome

Large bundled length of DNA that contains many genes. Large sections of DNA do not code for genes. Found in the nucleus of the cell.

Centromere

Area where the chromatids of a chromosome are attached

Homologous chromosomes

Pair of chromosomes that are the same size, same appearance and have the same genes. One from mother, one from father.

Explanation: Get duplicated before cell division, so i and I (i and l are homologous chromosomes) become ii and ll. They are still homologous chromosomes, just doubled. These doubled chromosomes are each sister chromatids (i is a sister chromatid to i). They are then halved the opposite way (cutting chromatids each in half) during anaphase.

Gonads

Ovaries and testes; organs that produce sex cells

Gametes

Sex cell, produced by gonads (sperm and egg)

Somatic cells

Any cells in the body other than reproductive cells

Stages of cell division

Prophase, Metaphase, Anaphase, Telophase (PMAT)

How do mitosis and meiosis differ?

Meiosis goes through mitosis twice, results in 4 haploid gametes. Mitosis results in only two diploid cells.

Mitosis

Begins with one duplication of DNA/chromosomes. Cell division in somatic cells, resulting in 2 identical daughter cells. Used for growth and repair.

Sex chromosomes

Chromosomes that determine the sex of an individual, 23rd pair. XX=Female, XY=Male.

Autosomes

Any chromosome that is not a sex chromosome; first 22 pairs

Phases of mitosis simplified

Prophase - chromosomes condense and become visible

Metaphase - chromosomes line up in the middle of the cell

Anaphase - sister chromatids separate and move to opposite poles

Telophase - new nuclear membranes form around the separated chromosomes, creating two new cells

Mutagen examples

Radiation (UV, Nuclear, X-rays), chemicals such as asbestos, tobacco, and benzene in pesticides, infectious agents like HPV

Germline mutations

mutation in a gonads that produces gametes, can be passed on to children

Somatic mutations

Mutations that occur in body cells, aren't passed to offspring, and don't affect the gametes

Point mutations

Change in nitrogenous bases, possibly changing the amino acid order. Each point mutation usually only affects 1 gene. Types are substitution, insertion and deletion.

Point mutations - Substitution

One nucleotide is swapped for another, only changes DNA code for one amino acid. Example is sickle cell anaemia.

Turner syndrome

A chromosomal disorder in females in which either an X chromosome is missing, making the person XO instead of XX, or part of one X chromosome is deleted.

Trisomy vs Monosomy

trisomy- extra chromosome
monosomy- missing chromosome

Point mutations - Insertion

Extra nucleotide/s are inserted, changing the DNA sequence for all following amino acids. This is called a frameshift mutation. Example is fragile X syndrome

Point mutations - Deletion

A nucleotide is deleted from a sequence, changing all following amino acid coding. Another frameshift mutation. Example is Duchenne muscular dystrophy.

Chromosomal mutations

Affects multiple genes. As chromosomes occur in pairs, if one is mutated the other is likely to be normal.

Chromosomal mutation types

Duplication - part copied (increases gene expression)

Inversion - a segment is removed, then replaced within the chromosome in reverse order

Deletion - part deleted

Insertion - A portion of one chromosome is removed and replaced in another chromosome.

Translocation - Segments of two chromosomes are exchanged

How does a monosomy occur during meiosis

Homologs fail to separate during meiosis 1

Nondisjunction

An error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other.

How does a trisomy occur?

A trisomy occurs when two chromosomes join because of an error in cell division

Chromosomal disorders caused by non-disjuction

•Down syndrome (trisomy 21)

•Edwards syndrome (trisomy 18)

•Klinefelters syndrome (trisomy XXY)

•XYY syndrome (trisomy XYY)

•XXX syndrome (trisomy X)

•Turners syndrome (monosomy XO)

Other mutations don't make it past the embryo

Meiosis

A type of cell division that occurs in gonads. Starts with one duplication of DNA/Chromosomes. Results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes. Used for reproduction.

Mutation consequences

Beneficial increases survival and reproducing, deleterious may result in death and prevents genetics passing on, neutral leads to varied population.

Alleles

Alternative forms of a gene. In a homologous pair, the 2 chromosomes each have one variation of specific genes.

Co-dominance

situation in which both alleles of a gene contribute to the phenotype of the organism

Incomplete dominance

Due to a heterozygous individual; a blend of both alleles appears in the phenotype in heterozygous individuals

Carrier

Carries the recessive allele without displaying the trait, heterozygous, and can pass the allele to their offspring.

Pedigree genders

Circle is female

Square is male

Autosomal dominant

Every generation; each affected individual has affected parent

Autosomal recessive

Not every generation.

• if unaffected parents and affected child, it must be recessive

• if both parents affected, all children must be too

X-linked dominant

• FDD - Affected father = all affected daughters (and mother).

• Unaffected mother cannot have affected sons or father as she can’t be a carrier and husband can only pas to daughters

• mostly affects females

X-linked recessive

• MSR - Affected mothers = all affected sons (and father, and mother must at least be a carrier)

• Unaffected mother can have affected sons (she can be a carrier)

• More common in males

Karotype

Complete view of chromosomes in homologous pairs

Heterozygous vs homozygous

Heterozygous - One dominant and one recessive allele (Bb)

Homozygous - 2 dominant or 2 recessive alleles (BB or bb)

Different dominance types

Simple dominant/recessive, sex-linked, co-dominance, incomplete dominance.

Sex-linkage

While the Y chromosome determines gender, it is small and therefore has few equivalent genes with the X chromosome. The X chromosome therefore often has full ‘decision’ over certain characteristics, as there is no overriding genes (in males). Represented like: X^BX^b  and X^BY^-

Red blood cells (RBCs)

Have antigens (proteins) on their surface to determine our blood type. Types are A, B, AB, and O. Also either does or doesn’t have Rh proteins to determine positivity or negativity.

What are the 3 alleles for the ABO blood type gene?

•I^A – Antigen A, (type A blood), dominant

•I^B – Antigen B (type B blood), dominant

•i – No antigen (type O blood), recessive

I^A and I^B are dominant over i but have equal dominance over each other.

Why does the body produce antibodies for certain blood types and not others?

Depending on the blood type that your body is familiar with (the blood type you have), your body creates antibodies for the blood type it does not know.

A blood - B antibodies

B blood - A antibodies

AB blood - No antibodies

O blood - A and B antibodies

Rhesus factor

Another antigen that either is or is not on your RBCs. Also known as the D antigen, written as Rh+ or Rh-. If you have the D antigen, your blood type is positive, if you don’t, it’s negative.