7. Patterns of Inheritance

PATTERNS OF

INHERITANCE

PLAN OF LECTURE1. GENETICS and HUMAN GENETICS2. MAIN CONCEPTS of GENETICS3. MENDEL'S LAWS of INHERITANCE4. MONOGENOUS DISEASES 5. MENDELIAN TRAITS IN HUMANS

STUDENTS' INDEPENDENT STUDY PROGRAM

Objectives for Students' Independent Studies1. General genetics and human genetics as sciences.2. The main terms of genetic.3. The classification of cross: monohybrid, dihybrid,

polyhybrid and test cross.4. Patterns of inheritance. 5. Mendel's Laws of inheritance6. Mendelian Traits in humans.7. Monogenous diseases.Applicable literature materials:Medical biology / K. L. Lasarev – Simferopol: IAD CSMU,

2002. p. 114 - 123.

KEY WORDS AND PHRASES: GENETIC, ANTHROPOGENETIC, HEREDITY, INHERITANCE, GENE, GENOTYPE, PHENOTYPE, GENOME, ALLELE, LOCUS, HETEROZYGOUS, HOMOZYGOUS, GENETIC ANALYSIS, DOMINANT CHARACTER, RECESSIVE CHARACTER, MONOHYBRID CROSS, POLYHYBRID CROSS, TEST CROSS, MENDELIAN TRAITS, MONOGENOUS DISEASES.

• Genetics is the science that learns the peculiarities of the hereditary and variability, which are the main characteristics of living things.• Human genetics (anthropogenetic) is the science that learns the

peculiarities of hereditary and variability in human organism.

2. MAIN CONCEPTS of GENETICS

• Heredity - is the transmission of characteristics from parent to offspring through the gametes.• Gene - a unit of heredity, a segment

of a DNA that contains all the information required for synthesis of polypeptide chain. Each gene has a specific position (locus) on a chromosome.

Allele - an alternate form of a gene, that

can occupy a particular chromosomal locus. In humans and other diploid organism there are two alleles, one on each chromosome of a homologous pair.

An individual who has two identical alleles for a gene at a particular locus on homologous chromosomes is homozygous for that gene. An individual with two different alleles at a particular locus on homologous chromosomes is heterozygous.

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.

• Genotype- is the genetic constitution of an organism (a diploid set of genes).

• The genotype describes the organism's alleles, whole the phenotype describes the outward expression (physical appearance of an individual) of an allele combination.

• The phenotype of an individual is the observable outward manifestation of the genes that it carries.

• Genome - is a collection of genes of an organism in cells (may be a haploid set of genes).

THE GENERAL GENETIC SYMBOLS OF SOLVING THE SITUATIONAL TASKS:

1) By Latin letters (А, а)mark alleles genes, which reply to alternative traits (signs).

2) The parent generation is called as "P".

3) Female genotype is written first and has the symbol ♀ (the mirror of Venus, the godess of beauty);

4) the male genotype the symbol ♂ is used (the shield and the spear of Mars - god of war). 5) The interbreeding is marked by the sign of multiplication (x).6)The offsprings are called as letter F (Lat. filii) with numeral index, which belongs to number of the generation ( F1 - children, F2 -grandchildren, etc.)

MONOHYBRID CROSSES

P

F1

F2

parents

f i l i i

×

f i l i i

АА аа

THE CROSSES THAT INVOLVE ONLY ONE TRAIT (ONLY ONE PAIR ALLELES) ARE CALLED MONOHYBRID CROSSES.

THE CROSS BETWEEN INDIVIDUALS, WHICH DIFFER IN TWO VARIOUS TRAITS (TWO PAIRS ALLELES) ARE CALLED DYHYBRID CROSSES (TWO PAIRS GENES IN TWO PAIRS OF CHROMOSOMES - NOT LINKED INHERITANCE).

3. MENDEL'S LAWS of INHERITANCEI. The law of monotony of the first filial generation:DURING CROSSING TWO HOMOZYGOUS WHICH ARE DIFFER FROM EACH OTHER BY ONE TRAIT ALL PROGENY IN THE FIRST FILIAL GENERATION IS MONOTONY AS WELL AS PHENOTYPIC AND GENOTYPIC.

LAW OF MONOTONY

P parents

F1

×AAaa

Aa

G: Aa

♀ ♂

II. THE LAW OF SEGREGATION:

STATES THAT 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.

LAW OF SEGREGATION

P

G:

F2

3 : 1

parents ×Aa Aa

A, aA, a

AA Aa aaAa

♂

♀

III. LAW OF INDEPENDENT ASSORTMENT

Based upon the results of the dihybrid cross, Mendel proposed what is now know as THE LAW OF INDEPENDENT ASSORTMENT, WHICH STATES THAT A GENE FOR ONE TRAIT DOES NOT INFLUENCE THE TRANSMISSION OF A GENE FOR ANOTHER TRAIT.

This is the classic dihybrid ratio, which indicates that both parent are heterozygous at both loci.

This ratio is the result of the two genes behaving completely independently of each other in the cross (not-linked inheritance).

MENDELIAN TRAITS in HUMANS

MENDEL'S LAWS APPLY TO ALL DIPLOID SPECIES. THE NATURE OF THE PHENOTYPE IS IMPORTANT WHEN-EVALUATING TRANSMISSION OF MENDELIAN TRAITS. IN HUMANS, DISORDERS OR TRAITS CAUSED BY A SINGLE GENE ARE CALLED MENDELIAN TRAITS.

MENDELIAN TRAITS in HUMANS

• Brachydactyly - Short fingers.• Polydactyly - Extra fingers and

toes.• Huntington's disease -

Degradation of nervous system, starting in middle age.

SOME DOMINANT TRAITS IN HUMANS:

Brachydactyly

Polydactyly

Huntington's disease

SOME RECESSIVE TRAITS IN HUMANS:

• Albinism - Lack of melanin pigmentation.• Alkaptonuria - Inability to

metabolize homogenic acid.• Duchenne muscular dystrophy

- Wasting away of muscles during childhood.

Albinism - Lack of melanin pigmen-tation.

Alkaptonuria - Inability to metabolize homogenic acid

Duchenne muscular dystrophy