Mendel's laws 31 1 2011

1. Mendel’s Laws Of Inheritance Vasanthan V 09MTBI29

2. Gregor Johann Mendel 1822 - 1884 Austrian monk Experimented with pea plants He thought that ‘heritable factors’ (genes) retained their individuality generation after generation Principles of genetics were developed in the mid 19th century Experimented with pea plants, by crossing various strains and observing the characteristics of their offspring Gregor mendel is the father of modern genetics.

3. Gregor Mendel–studied inheritance of seven traits in pea plants and first used the term dominance and recessiveness Proposed similar but separate inheritable characters, one from each parent, later to be called genes.

4. Inheritance Parents and offspring often share observable traits. Mendelian inheritance  (or  Mendelian genetics  or  Mendelism ) is a set of primary tenets relating to the transmission of  hereditary  characteristics from parent organisms to their offspring; it underlies much of  genetics . They were initially derived from the work of  Gregor Johann Mendel  published in 1865 and 1866 which was "re-discovered" in 1900, and were initially very controversial..

5. The units of inheritance are alleles of genes Traits are controlled by alleles - alternate forms of a gene Found on homologous chromosomes at a particular gene locus The dominant allele masks the expression of the other allele - the recessive allele Genotype refers to the alleles an individual receives at fertilization Homozygous - an organism has two identical alleles at a gene locus Heterozygous - an organism has two different alleles at a gene locus Phenotype - the physical appearance of the individual. 9- 5

6. Dominant and Recessive A dominant allele is represented by a CAPITAL letter. It is always expressed when present. (BB or Bb) A recessive allele is represented by lower case letter. It is only expressed when an individual has 2, one from mom and one from dad. (bb only)

7. Genotype and Phenotype A genotype is the genetic make-up of an individual, expressed in letters. (BB, Bb, bb) A phenotype is the physical appearance of an individual, determined by his or her genotype. (black, brown, short, tall, etc)

8. Homozygous and Heterozygous Homozygous : when both alleles of a genotype are the same (either both dominant, BB, or both recessive, bb) Heterozygous : when one allele is dominant and one is recessive (Bb only)

9. Punnett Squares Punnett Squares are used to show the mating of two parents and the possible offspring they can produce.

10. Mendel observed seven pea plant traits that are easily recognized in one of two forms:  1.        Flower color : purple or white 2.        Flower position : axial or terminal 3.        Stem length : Tall or Dwarf 4.        Seed shape : round or wrinkled 5.        Seed color : yellow or green 6.        Pod shape : inflated or constricted 7.        Pod color : green or yellow

11. True-breeding plants make offspring that are the same as the parents every time From his experimental data, Mendel deduced that an organism has two genes ( alleles ) for each inherited characteristic One characteristic comes from each parent!!! FLOWER COLOR FLOWER POSITION SEED COLOR SEED SHAPE POD SHAPE POD COLOR STEM LENGTH Purple White Axial Terminal Yellow Green Round Wrinkled Inflated Constricted Green Yellow Tall Dwarf Mendel’s seven pea characteristics

12. Mendel deduced the underlying principles of genetics from these patterns Dominance Segregation Independent assortment

13. Mendel’s data

14. 1.Mendel’s law of dominance If your two alleles are different ( heterozygous , e.g. Bb), the trait associated with only one of these will be visible (dominant) while the other will be hidden (recessive). E.g. B is dominant, b is recessive . B b B BB Bb b Bb bb Eggs Sperm

15. Contd.. Law of dominance :  In a hybrid union, the allele which expresses itself phenotypically is the  dominant  allele while the other allele which fails to express itself phenotypically is the  recessive  allele. The hybrid individual shows phenotypically only the dominant character. The law of dominance is often described as  Mendel’s first law of inheritance .

16. 2.Mendel’s law of segregation A normal (somatic) cell has two variants (alleles) for a Mendelian trait. A gamete (sperm, egg, pollen, ovule) contains one allele, randomly chosen from the two somatic alleles. E.g. if you have one allele for brown eyes (B) and one for blue eyes (b), somatic cells have Bb and each gamete will carry one of B or b chosen randomly . Law of segregation – the separation of alleles into separate gametes. B b B BB Bb b Bb bb Eggs Sperm

17. Contd.. The law of segregation states: Each individual has two factors for each trait The factors segregate (separate) during the formation of the gametes Each gamete contains only one factor from each pair of factors Fertilization gives each new individual two factors for each trait

18. Monohybrid cross What happens when true breeding plants with two distinct forms of a trait are crossed? Progeny show only one form of the trait. The observed trait is called dominant. The masked trait is called recessive . Progeny- A genetic descendant or  offspring

19. Contd.. When we are looking at one trait (such as eye color or height), we do a monohybrid cross . Parents are called P(Parental Generation) Children are called F1(First Fililal Generation) Grandchildren are called F2(Second Filial Generation)

20. Contd.. Two heterozygous parents produce gametes with T or t allele equally frequently. Offspring genotypes 1/4 TT : 1/2 Tt : 1/4 tt Offspring phenotypes 3/4 tall : 1/4 short

21. 3.Law of Independent Assortment "When a dihybrid (or a polyhybrid ) forms gametes, ( i ) each gamete receives one allele from each allelic pair and (ii) the assortment of the alleles of different traits during the gamete formation is totally independent of their original combinations in the parents.  In other words,   each allele of any one pair is free to combine with any allele from each of the remaining pairs during the formation for the gametes This is known as the  Law of Independent Assortment of characters .   It is also referred to as  Mendel’s third law of heredity.

22. Explanation of the law of independent assortment:   The principle of independent assortment was explained by Mendel with the help of a dihybrid cross involving characters of cotyledon color (yellow / round) and seed shape  (round / wrinkled). Mendel crossed a true breeding variety of pea having yellow cotyledons (YY) and round seeds (RR) with another true breeding variety having green cotyledons ( yy ) and wrinkled seeds ( rr ).

23. Thus, the yellow round parent has the genotype (YYRR) and the green wrinkled parent ( yyrr ). Since each parent is homozygous for both characters (color and shape), each will produce only one type of gametes. The (YYRR) parent will produce all (YR) type gametes and the ( yyrr ) will produce all (yr) type gametes.

24. Contd.. All F 1   dihybrids resulting from the fusion of these gametes would be double heterozygous with ( YyRr ) genotype and appear yellow round. This indicated that in the dihybrid cross also in each pair, the alleles behaved exactly in the same way as in the monohybrid cross. Both the dominants (Y and R) expressed themselves in F 1 while both the recessive alleles (y and r ) remained hidden.

25. Types of gametes formed by F 1  dihybrid :   According to Mendel, during gamete formation by the F 1  dihybrid , the alleles in both pairs Y-y and R-r first segregate from each other (Law of segregation). Each pair segregates independently of the pair. Then the alleles enter the gametes. A gamete can receive only one allele from each pair, i.e. Y or y and R or r.

26. Contd., Similarly, a gamete that receives a factor (gene) for color must also receive factor for shape (a factor for every character must be present in each gamete). Thus, a gamete that receives Y for color may receive R or r for shape. This would result in (YR) and (Yr) types of gametes. Similarly, a gamete that receives y for color may receive R or r for shape.

27. Contd.. This would give ( yR ) and (yr) types of gametes. In other words, the F 1   dihybrid would produce four types of gametes (YR), (Yr), ( yR ) and (yr) in equal proportions. This is the principle of independent  assortment  of characters. There will be four types of male gametes and four types of female gametes formed by the F 1   dihybrid .

28. 9:3:3:1

29. Contd.. During self-fertilization or inbreeding of the F 1   dihybrids to produce an F2generation, these male and female gametes can form maximum to dihybrid unions as shown in the  Punnet’s Checker-board . These can be grouped into four kinds on the basis of phenotypic appearance. i.e.  yellow round ,  yellow wrinkled ,  green round and green wrinkled  in the ratio of 9:3:3:1 respectively. This is called the  Phenotypic dihybrid ratio .

30. Contd.. The 16 squares of the checker board are serially numbered for convenience. The squares represent the 16 possible combinations of the gametes which might result. The genotypes and the phenotypes of the F 2  offspring are shown in the sixteen squares. A count of these squares shows the four kinds of phenotypes and their ratio in the F 2  generation.

31. Mendel came to four important conclusions from these experimental results:   1.   The inheritance of each trait is determined by "units " or " factors"  (now called genes) that are passed on to descendents unchanged. 2.   An individual inherits one such unit from each parent for each trait. 3.   A trait may not show up in an individual but can still be passed on to the next generation. 4.   The genes for each trait segregate themselves during gamete production.

32. Vasanthan V 09MTBI29