This document discusses genetics and inheritance of genetic diseases. It covers Gregor Mendel's experiments with pea plants which laid the foundations of genetics, including genes, alleles, dominant and recessive traits. It also discusses chromosomes, genetic crosses, sex-linked inheritance and examples of genetic disorders like cystic fibrosis and muscular dystrophy. The role of mutations in causing genetic diseases as well as the process of genetic counseling is summarized.

1. Inheritance and Genetic Diseases
Introduction: the History of Genetics
 Gregor Mendel considered father of genetics 1800’s monk
 Experiments on pea plants- self- pollinating flowers could easily be cross fertilized= hybrid
 HYBRID: new types of plants formed by cross fertilizing varieties of the same species
 He first grew many varieties of pea plants- made sure each was purebred (when it pollinates
itself the successive generation always looks like the parent plant)
 Mendel began to cross purebred plants that differed in only one characteristic=
MONOHYBRID CROSS and always chose contrasting traits (height of plant)
 To cross two plants, pollen was spread from the stamen of a tall plant to the stigma of a
short plant
 SCIENTISTS AT THE TIME thought that offspring were a blend of their parents- Mendel
disproves this
Mendel’s Results
 All the offspring of a cross look the same- show characteristics of only one parent
 Let the offspring self- fertilize= one or other trait showed in a 3:1 ratio
This Proved That:
 Genes that control characteristics occur in pairs
 If the genes are different, one will be DOMINANT and the other RECESSIVE
 During the formation of gametes, the paired genes separate and each gamete receives one
of the genes
Chromosomes, Genes and Alleles
 25 000 genes on a chromosome (carry hereditary genes)
 GENE: small piece of DNA in the chromosome which carries info about characteristic in the
body
 LOCUS: a particular position of a gene
 ALLELE: different forms of the same gene, found in the same position on the corresponding
homologous chromosome- one allele from mother and one from father
HOMOZYGOUS= alleles for particular characteristic are the same- gene is either TT or tt
HETEROZYGOUS= two alleles for same characteristic are different- gene is Tt
Genetic Diagrams
GENOTYPE: genetic factors present in organism
PHENOTYPE: visible expression of the genotype- way organism looks is due to genotype
MONOHYBRID CROSS: only one hereditary trait is investigated at a time

2. Symbols used in Genetic Diagrams
1. GENERATIONS
a. P- parent generation
b. F1- first Filial- first generation of offspring
c. F2- second Filial- second generation of offspring
2. ALLELES OF A GENE represented by capital and small letters
a. DOMINANT TRAIT= capital letter eg tall plant: T
b. RECESSIVE TRAIT= small letter eg dwarf plant: t
3. As there are 2 alleles for each trait (one on each chromosome in homologous pair) we write
two letter- DOMINANT ALLELE ALWAYS COME FIRST
a. Homozygous (purebred) tall plant= TT (both alleles tall)
b. Homozygous (purebred) dwarf plant= tt (both allele dwarf)
c. Heterozygous (hybrid) tall plant= Tt (one allele dwarf, one tall)
4. DURING MEIOSIS in the formation of gametes, homologous chromosomes separate. Each
gamete receives only 1 allele of pair. Eg if parent is Tt, gamete receives either T or t
LOOK AT GENETIC DIAGRAM AND LEARN/ PRACTICE SOLVING GENETIC PROBLEMS
NOTE: always include key
In Genotype say the literal genes present eg TT; tt; Tt; Tt
In Phenotype, say the result of these genes eg Tall, Tall, Tall, Short (3 tall 1 short ) and say the
percentage (75% tall, 25% short)
The Inheritance of Sex
 The two gonosomes (23rd chromosome/sex chromosome) carry info that determine whether
offspring m/ f
 Female chromosome(X) never swops info with male chromosome (Y)
o Therefore info pertaining to sex including traits of that sex is inherited by offspring
as a complete set of info
 Y dominant over X, therefore all males carry XY all females carry XX
 This means father determines sex as mother donates X and father donates X or Y
 X chromosome is much longer and can carry many alleles
 Only small part of X and Y chromosomes can pair up during meiosis and no crossing over
occurs
 Alleles carried on the non-homologous part of X chromosome are called x linked/sex linked
alleles
 Men only have one X chromosome so will have one of each sex linked allele, this is why
certain genetic diseases are present more in males than females
- If gene on X chromosome mutates/gene for disease male will express it, female only
express it if gene is recessive on both X chromosomes

3. Genes, Alleles and Mutations
When do MUTATIONS occur?
 DNA strand of a gene changes forming a new allele. Change in DNA structure changes
information allele gives to the cell
o Chromosomal- damage to chromosome due to UV, cosmic rays, X ray, Radiation
o Point- single pair of nucleotides in certain point in DNA replaced by different base
pair
 Substitution- wrong nucleotide
 Deletion- less
 Insertion- extra
Somatic Mutations
 Occur in somatic body cells- kidney/ bone skin
 Damage/ kill/ convert into tumour cells= cancerous
 when mitosis happens, Mutation transferred to daughter cells in tissue/ organ
 METASTASIS(spread of disease from part to another non-adjacent part.) occurs when cancer
cells spread through body
 Somatic mutation dies when cells die/ tumour cells are killed
 Occurs after conception/ will not be passed down
Germline Mutations
 Occur in egg/ sperm- passed onto zygote which will have mutation present in all cells
 Next generation of gametes will carry mutation= FIXED MUTATION/ DRIVES EVOLUTION

4. Beneficial/ Harmless/ Harmful
Beneficial Mutations
 Natural selection based on principle that genes mutate & form new alleles
 Depending on environmental changes, mutation can become harmful/ beneficial
Harmful Mutation
 Change gene so that allele cannot function (albinism) change message gene gives (extra
limb)
 Cause disease/ death
 LETHAL ALLELE- gene is not coding for production of important substance
Harmless
 Also called neutral/ changes physical characteristics without affecting body function
Cystic Fibrosis- a Genetic Disorder
 Lethal allele- cannot produce protein in cells of lungs to transport salts/ water across
membrane into cells
 Salt/ water collect in air passages= sticky mucous blocking air passage in/ out of lungs
 O2 deprived/ CO2 poisoning/ lung infections/ organ failure
 Children of 2 carrier parents have a one in four chance of being sufferers (25%)
Sex Linked Allele
 Certain characteristics/ genetic diseases more prominent in m than f
o Due to structure of sex chromosomes (heterosomes)
 X CHROMOSOME- many genes on it/ Y CHROMOSOME- smaller/ few genes on it
 If a gene on X mutates, mutation will be seen in male because he has only one gene for that
trait (on his X)
 If mutation is recessive, it will only be seen in female if both X chromosomes have that trait
Muscular Dystrophy
 Duchenne muscular dystrophy occurs when gene on X fails to make muscle protein
dystrophin
 Begins in early childhood- progressive loss of muscle strength- die in 20’s of respiratory/
cardiac muscle failure
Colour Blindness
 Red green both recessive and sex linked
 Recessive gene for colour blindness found on X, Y doesn’t carry gene for colour distinction
 Carrier female will be normal as dominant normal gene on other X will dominate
 BUT if she has a son, he can inherit her recessive X in which case he will be colour blind
 If a girl receives a recessive gene from both Barents, she will be colour blind

5. Genetic Counselling
Couple who want kids but have genetic disease in family go for genetic counselling
STEP ONE
 Family tree drawn up to find chance of child inheriting disease
STEP TWO
 Mother goes for genetic screening- find if she has defective allele
 DNA placed in solution with radioactive DNA that attached to mutant gene GENEPROBE
STEP THREE
 If they are at risk, counsellor discusses % chance of having sufferer child, effects of disorder
on child, other children/ own lives
 Couples RELIGIOUS/ MORAL/ CULTURAL beliefs about abortion discussed
OPTIONS
 No children
 Have pregnancy and abort if foetus is affected
 Have child regardless of outcome