Theme: Organism’s level of organization of genetic

information. Gene ineractions.

Chromosomal theory of heredity. Genetics of sex

Lecturer: ass. prof. Tatyana Bihunyak

Introduction to Genetics

• GENETICS – branch of biology that deals with heredity and variation of organisms

• Chromosomes carry the hereditary information (genes)

• Arrangement of nucleotides in DNA

• DNA RNA Proteins

Gregor Johann Mendel• Austrian Monk, born in what is now

Czech Republic in 1822• Son of peasant farmer, studied

Theology and was ordainedpriest Order St. Augustine.

• Went to the university of Vienna, where he studied botany and learned the Scientific Method

• Worked with pure lines of peas for eight years

• Prior to Mendel, heredity was regarded as a "blending" process and the offspring were essentially a "dilution"of the different parental characteristics.

•Mendel looked at seven traits or characteristics of pea plants

• Mendel was the first biologist to use Mathematics – to explain his results quantitatively.

• Mendel predicted

The concept of genes

That genes occur in pairs

That one gene of each pair ispresent in the gametes

• Human genetics is the science that learns the peculiarities of the hereditary and variability in human organism

• Heredity – is the transmission of characteristics from parent to offspring through the gametes

Genetics terms you need to know:

• Inheritance – is the way of passing of hereditary information which depends on the forms of reproduction

During asexual reproduction the main traits are inherited through spores or vegetative cells, that's why the maternal and daughter cells are very similar.

During sexual reproduction the main traits are inherited through gametes.

Genetics terms you need to know:

Genetics terms you need to know:Gene – a unit of heredity; a section of DNA

sequence encoding a single proteinGenotype – is the genetic constitution of an

organism (a diploid set of genes)Genome – is a collection of genes of an

organism in sex cells (a haploid set of genes)

Alleles – two genes that occupy the same position on homologous chromosomes and that cover the same trait (like ‘flavors’ of a trait)

Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located

Homozygous – having identical genes (one from each parent) for a particular characteristic (AA or aa).

Heterozygous – having two different genes for a particular characteristic (Aa).

Dominant – the allele (A) of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition.

Recessive – an allele (a) that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous.

Genetics terms you need to know:

• Genotype – describes the organism’s alleles (the genetic makeup of an organisms)

• Phenotype – the physical appearance

of an organism (Genotype + environment)

• Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait

P = Parental generation

F1 = First filial generation; offspring from a genetic cross

F2 = Second filial generation of a genetic cross

1. The law of monotony of the first filial generation

A - yellow seed; a - green seed

P: ♀ AA x ♂ aaG (Gametes): A aF1: Aa (yellow)

During crossing two homozygous which are differ from each other by one trait all progeny in the first filial generation is monogyny as well as phenotypic and genotypic

2. The law of segregation A cross between plants obtained from F1 plants.P: ♀ Aa x ♂ AaG: A, a A, a F2 : AA; Aa; Aa; aa

From a pair of contrasting characters (alleles) only one is present in a single gamete and in F2 these characters are segregated in the ratio of three to one (3:1) by phenotype and 1:2:1 by genotype.

When gametes are formed in heterozygous diploid individuals, the two alternative alleles segregate from one another.

Dihybrid crosses• Matings that involve parents

that differ in two genes (two independent traits)

For example, flower color:

P = purple (dominant)

p = white (recessive)

and stem length:

T = tall t = short

Dihybrid cross: flower color and stem length

TT PP tt pp (tall, purple) (short, white)

Possible Gametes for parents

T P and t p

F1 Generation: All tall, purple flowers (Tt Pp)

TtPp TtPp

TtPp TtPp

tp tp

TP

TP

Dihybrid cross F2

If F1 generation is allowed to self pollinate, Mendel observed 4 phenotypes

Tt Pp Tt Pp (tall, purple) (tall, purple)

Possible gametes:TP Tp tP tp

Four phenotypes observedTall, purple (9); Tall, white (3); Short, purple (3); Short white (1)

TTPP TTPp TtPP TtPp

TTPp TTpp TtPp Ttpp

TtPP TtPp ttPP ttPp

TtPp Ttpp ttPp ttpp

TP Tp tP tp

TP

Tp

tP

tp

Genotype ratios (9): Four Phenotypes:1 TTPP2 TTPp2 TtPP4 TtPp1 TTpp2 Ttpp1 ttPP2 ttPp1 ttpp

Dihybrid cross: 9 genotypes

Tall, purple (9)

Tall, white (3)

Short, purple (3)

Short, white (1)

Principle of Independent Assortment

• Based on these results, Mendel postulated the 3. Principle of Independent Assortment:

“Members of one gene pair segregate independently from other gene pairs during gamete formation”

Genes get shuffled – these many combinations are one of the advantages of sexual reproduction

Incomplete Dominance

Snapdragon flowers come in many colors.

If you cross a red snapdragon (RR) with a white snapdragon (rr)

You get PINK flowers (Rr)! R R

R r

r r

Genes show incomplete dominance when the heterozygous phenotype is intermediate.

Incomplete dominance

Incomplete Dominance

When F1 generation (all pink flowers) is self pollinated, the F2 generation is 1:2:1 red, pink, white

R R R r

R r

r r

R r

R

r

Multiple Alleles• Genes can be characterized by more than 2

alleles • Multiple alleles (>2) can be studied only in

populations, because any individual carries only 2 alleles at a particular locus at one time

• ABO blood groups – Each individual is A, B, AB, or O phenotype – Phenotype controlled by isoagglutinogen

marker on RBC– IA and IB alleles are dominant to the IO allele – IA and IB alleles are codominant to each

other

Codominance

• Both alleles are equally dominant

• Heterozygotes express both alleles = distinct expression of the gene products of both alleles can be detected

• ABO blood group

PhenotypePossible

Genotype

Antigen on RBC surface

Antibody Made in Plasma

A IAIO, IAIA A Anti-B

B IBIO,IBIB B Anti-A

AB IAIB AB Neither

O IOIO O Both

Continuous variation

• Continuous variation (polimery). Different dominant non-allele's genes affect on one trait, making it more expressive.

• Traits determined by more than one gene are polygenic - meaning "many genes" - or quantitative traits.

Continuous variation

• Usually, several genes each contribute to the overall phenotype in equal, small degrees.

• The combined actions of many genes produce a continuum, or continuously varying expression, of the trait.

• Example: Skin color is familiar example of polygenic trait in humans.

Skin color is quantitative trait, that are controlled by two pairs of genes A1a1, A2a2.

Let’s sign skin color as • A1A1A2A2 – very dark• A1A1A2a2 or A1a1A2A2 — dark• A1a1A2a2 — medium brown • A1a1a2a2 or a1a1A2a2 — light• a1a1a2a2 — white (pale skin).

TASK

A woman with white skin married a medium-brown man. What is the skin

color possible for their children?

P: ♀a1a1a2a2 ♂ A1a1A2a2

G: a1a2 A1A2, A1a2, a1A2, a1a2

F1: A1a1A2a2; A1a1a2a2; a1a1A2a2; a1a1a2a2

•Genes are located on the chromosomes.

•In human karyotype there are 46 chromosomes. In human diploid number there are thousands of different genes.

•Many genes may be present on the same chromosome. Such genes are said to be linked, or to constitute a linkage group.

•Linked genes were discovered by great American geneticist Thomas Hunt Morgan of Columbia University in 1910.

Unlinked Genes

Linked Genes

Thomas Hunt Morgan (1866-1945)

Nobel prize in 1933 for his research on the fly Drosophila in linkage and crossing-over, which he used to map the linear arrangement of genes along the chromosome.

T. H. Morgan studied chromosomes of Drosophila melanogaster (fruit-fly)

The fruit-fly was selected because 1) it breeds rapidly, attaining maturity in twelve days; 2) 30 generations can be bred in one year; 3) it has only eight chromosomes.

L = long wingsl = short wings

G = gray body g = black body

Autosomal Linkage. Dihybrid Testcross

Distance between the linked genes is measured in centimorgans (cM) or map units.

1 cM = 1 map unit = 1% of crossing over = 1% crossover gametes = 1% recombinants

Linkage between genes on the same chromosome

1) complete linkage - when genes stay together at a very short distance on the chromosome; person with complete linked genes can form only noncrossover gametes.2) incomplete linkage - when genes stay together at a far apart (under 50 cM); person with incomplete linked genes can produce crossover and noncrossover gametes, because during meiosis crossing-over takes place.

Crossing-over

Sex DeterminationSex DeterminationSex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

Sex DeterminationSex Determination

X X

X X

X y

X y

X y

X

X

Other Sex DeterminationSex Determination

• The Y chromosome sometimessometimes does not dictate its maleness –Absence of a second X

• XY fruit fly is male • XXY fruit fly is female

Environmental Sex DeterminationSex Determination

• Sex may be determined after fertilizationafter fertilization

• Determined by temperaturetemperature during early embryonic development – Turtles produce more females at a higher

temperature – Alligators and many lizards produce more males

at a higher temperature

Environmental Sex DeterminationSex Determination

• Sex may be determined after fertilizationafter fertilization

• Determined by temperaturetemperature during early embryonic development – Turtles produce more females at a higher

temperature – Alligators and many lizards produce more males

at a higher temperature

1. Allelic genes:A. Have the different loci in the homologous chromosomes.B. Are situated in the non-homologous chromosomesC. Are situated in the homologous chromosomes.D. Have the same loci in the homologous chromosomes.E. Have the different loci in the heterochromosomes.

2. Which type of the human ABO blood group is example of codominance:

A. I (O) B. II (A) C. III (B) D. IV (AB) E. All of the above.

3. How many linkage groups does human have?A. 46. B. 44. C. 23. D. 2. E. 1.

4. Genes A, B and C belong to one group of linkage. It is known, that the distance between the genes A and B is equal to 5 centimorgans and betweeen the genes B and C – 3 centimorgans. What is distance between the genes A and C?

A. 5 centimorgans. B. 3 centimorgans. C. 8 centimorgans.D. 2 centimorgans. E. 13 centimorgans.

“He who likes to eat fruit must climb the tree”

(English proverb)