Mendelian Genetics - Heredity Part II

Introduction to heredity

gene

a section of dna at a specific location on the chromosome

contains information that determines a trait

mendelian trait

single trait determined by 1 gene

somatic cell

any cell in the body apart from gametes

Gamete

Sex cell

Hereditary traits and Genetics

hereditary trait

characteristic that can be passed on from one generation to another

examples:

wet or dry earwax

attached / detached earlobe

skin colour

hair colour

eye colour

blood type

face shape

chin shape

ability to roll tongue

Monohybrid Inheritance

inheritance of 1 characteristic with 2 contrasting forms controlled by a single gene

each gene is made up of a pair of alleles

alleles can be dominant or recessive

homologous chromosomes

exist in pairs where

one chromosome comes from the male parent

one chromosome comes from the female parent

both have the exact same gene loci

gene

unit of inheritance found on a particular locus of a chromosome

small portion of DNA in a chromosome that controls a particular characteristic or protein in an organism

alleles

alternative version of a gene that occupy the same locus on a pair of homologous chromosomes

can exist in 2 forms:

dominant

dominant allele will express itself in both homozygous dominant and heterozygous conditions

recessive

recessive allele will only express itself in a homozygous recessive genotype

Phenotype

traits of an organism that can be seen

influenced by genotype + environment

Genotype

genetic make-up of an organism that is inherited from its parents

Homozygous organism (for a certain trait)

happens if the 2 alleles controlling the trait are the same

possible combinations:

homozygous dominant (TT)

homozygous recessive (tt)

Heterozygous organism (for a certain trait)

happens if the 2 alleles controlling the trait are not the same

possible combination: Tt

Mendel's Monohybrid Experiment

Mendel cross-bred pure-bred Tall pea plants with pure-bred dwarf plants

F1 Generation hybrids all turned out to be tall plants

After the F1 offspring self - fertilized and produced seeds, F2 Generation plants came up

Ratio of tall plants to dwarf plants: 3 : 1(approx)

why?

genes are responsible for this

each charactersitic is controlled by a pair of factors in the cells

If the 2 factors are different, only the dominant factor will express itself

both the tall and short plants are pure bred

however, only the tall alleles were dominant and the short alleles were recessive

so when the 2 dominant tall gametes fused with the 2 recessive short gametes, the dominant alleles were expressed in heterozygous conditions while the recessive alleles were only expressed in homozygous conditions

this resulted in 1 dwarf and 3 tall hybrid plants

Genetic models

can be used to explain how alleles are passed on to offspring

predict the traits that will be displayed by the offspring

Sex Determination in humans

sex chromosomes

chromosomes that determine the sex of an organism

Autosomes

chromosomes in cells other than the sex chromosome

2 types of sex chromosomes:

X chromosome

Y chromosome

sex cells

cells that produce gametes by meiosis

somatic cells

other cells in the body

humans have:

22 pairs of autosomes

1 pair of sex chromosomes in each cells

XY - male

XX - female

male gametes

contain either the X or Y chromosome

Female gametes

contain only the X chromosome

how is the sex of the zygote determined?

if an X - carrying sperm fertilizes the ovum

the zygote will be female

if a Y - carrying sperm fertilizes the ovum

the zygote will be male

equal chance that the offspring could be male or female

Variation

differences in traits between individuals of the same species

traits of an individual are dependent on interactions between the genes and the environment

genetic variation

inheritable

variations due to environment

not inheritable

types of variations

continuous

deals with a range of phenotypes

controlled by many genes

occurs when 2 or more genes contribute to the final phenotype

genes show additive effect

alleles of a single gene combine and the combined affects show out as the phenotype

affected by environmental conditions

examples:

height

skin colour

caused by different levels of melanin in our skin

more alleles for melanin, darker skin (vice versa)

discontinuous

deals with a few clear-cut genotypes

genes do not show additive effect

not affected by environmental conditions

examples: eye colour, blood group

may arise due to:

crossing over and independent assortment of chromosomes and during meiosis

mutation in genetic material

provides new alleles to the gene pool for natural selection to act on

genetic variation is important to help organisms adapt and survive in changing environments

Mutations

change in structure of a gene (sickle - cell anemia) or in the chromosome number (Down's syndrome)

occurs as a result of error during the replication of the gene or chromosome

somatic mutations

occur in normal body cells

cannot be inherited

mutations can be inherited by the next generation if they occur in cells that give rise to gametes

dominant mutations

easily detected

recessive mutations

may not be detectable for generations

people will be carriers of it without anyone's knowledge

factors that increase the rate of mutations

radiation

chemicals

chromosome mutations

change in structure or number of chromosomes

example

Down's syndrome

aka Trisomy 21

people with this syndrome have an extra copy of chromosome 21

they have 47 chromosomes on total

normal humans have 46

how does this happen?

in rare cases when one of the eggs has 2 copies of chromosome 21

zygote formed has 3 copies of chromosome 21, so the child will have Down's syndrome

chromosome mutation in the gametes of a female parent can produce a child with Down's syndrome

gene mutations

change in the structure of DNA of a gene

produces variation between individuals as it results in new alleles of genes

examples

albinism

caused by a mutation in a recessive allele

absence of the pigment melanin results in reddish - white skin, white hair and pink eyes

albinos get sunburnt easily as they are very sensitive to sunlight

they can't look at the sun directly

sickle - cell anemia

caused by mutation in the gene controlling haemoglobin production

mutated gene is recessive, so it's only expressed in homozygous recessive conditions

how is the person affected?

sickle - shaped red blood cells have low oxygen carrying capacity and tend to clump together (and block blood flow)

fatal disease - sufferers usually die young

malaria caused by plasmodium

modifies red blood cells to obtain nutrients, escape destruction by splees

heterozygotes with both sickle - shaped and normal cells have an advantage

they are protected from malaria

they won't die due to lack of oxygen

such people are common in areas where malaria is prevelant like West Africa

Mutation and Selection

mutations can be:

harmful

individuals with harmful mutations will be eliminated

beneficial

individuals with beneficial mutation on the other hand may leave more offspring than normal individuals

nature "selects" organisms with more favourable characteristics to survive and reproduce

mutagens

can greatly increase the rate of mutations

rate of spontaneous mutatopn is extremely low in mutagens' absence

examples

radiation

UV light

X - ray

alpha and beta radiation

gamma rays

chemicals

mustard gas

formaldehyde

lysergic acid dielthylamide (LSD)