Laws Of Heredity (1963)

when we say she has her mother's eyes we mean that the little girl has inherited certain characteristics from her mother we say like begets like how is it that the mother cat has Offspring like this or this but this cat will never have Offspring like this how is it that mice like these continue to produce only gray mice for Generation after generation and how can these mice continue breeding only white mice for Generations there must be some mechanisms by which characteristics are transmitted from generation to generation in a consistent way perhaps we can learn something about these mechanisms of heredity by observing the results of mating Mice from two pure breeding strains that differ in color of fur we call this a hybrid cross the first generation of this cross looks like this each individual is gray does this mean that inheritance is determined only by by the gray parent what happens if two adult individuals of this hybrid generation are M the gray color characteristic is somehow dominant yet we can see that in this second generation inheritance from the original white parent is expressed in one individual the same kind of result occurs whether the original parents were white male and gray female or gray male and white female gray fur color predominates but white is expressed in the second generation how does this come about we will assume that in each parent there are precise specifications that determine the animals characteristics we call these specifications genes let us consider the genes that determine color of fur when a gene such as one for whiteness of fur has an alternative form such is one for greyness of fur the alternative form is called an alil now let's assume that the sex cells of the parents each contain one of these genes for color of fur if the sex cells unite the resulting cells or zygotes each contain a pair of genes for color of fur one gene for white and one for gray but we know that all the zygotes of this generation develop into grey mice it appears that only one of the genes for color is expressed in individuals of this generation and it is always the gray Gene in this case we would say that the gene for gray color is dominant over its alil for white color what happens when mice of this generation are M we believe that in each parent half the jeans are for Pure gray color of fur half for pure white we assume that the genes in each pair separate when sex cells are formed so that each sex cell contains only one gene for color of fur can we also assume that zygotes of the new generation would be formed through a chance combination of a gene from each parent a simple demonstration will show what to expect for each genan pair we will use one gray ball and one white ball we put the same number of pairs into each container and mix them well the bowls in the containers represent genes that can be transmitted by each parent what happens when such genes are combined purely through chance our random selection from the gray and white jeans of each container results in four possible combinations of pairs approximately one qu of the pairs are gray gray approximately one qu are gray white one qu white gray and one quar white white now if our ideas hold true we would expect the progeny of hybrid mice to contain pairs of genes for fur color like this one quarter would contain paired gray jeans and would be gray two quarters with Gene pairs consisting of one gray and one white would be gray since the gray Gene is dominant one quarter with white genes paired could only be white such genes expressed only when paired with each other never when paired with a dominant Al are called recessive genes thus we'd expect three4 of the mice of this generation to be gray and one quarter to be white how do the actual results of such crosses among live mice compare with our predictions the hybrid mice in our laboratory so far have produced young of which 29 are gray and 11 are white this is very close to our predicted ratio of 3/4 gray to one4 White the slight discrepancy can be attributed to the smallness of the sample do our ideas about heredity apply to other organisms as well as mice for example this we can take Garden pea plants pure breeding for white flowers and cross them with plants that have pure breeding for red flowers each parent has genes for color of flower if each parent contributes one gene for color of flower to The Offspring each plant of the new generation would have one gene for red color and one for white the actual results of such a cross show this since the gene for red is expressed we believe that the red Gene is dominant over the white one can we predict the results of self- fertilization in these hybrid plants if the same mechanisms of independent segregation and chance combination apply to genes for the color of flowers we would get Gene combinations like this since we believe the red Gene to be dominant we would expect the color of flowers in this generation to be 34s red and one qu white and this is in fact what we get approximately 3/4 of the progyny of the hybrids show the dominant trait red color and the remaining quarters show the recessive trait white color Garden peas can be used to demonstrate the inheritance of many other characteristics for example a tall variety can be crossed with a dwarf variety the resulting hybrids show tallness to be dominant and dwarfness to be recessive if these hybrids are self- fertilized we can predict how the dominant and recessive traits will be expressed so far we have considered inheritance in plants that differ in only one characteristic let us examine a cross between between plants that differ in two characteristics a DI hybrid cross one plant is a variety that is tall with white flowers the other is dwarf with red flowers the first generation of this di hybrid cross looks like this the dominant genes for tallness and for red color are expressed in this generation can we predict how these genes will be expressed if the DI hybrids are self fertilized let's go back to the original parents and their Gene pairs one has genes for tallness and white color the other has genes for dwarfness and red color if their sex cells are combined the resulting combinations can only be like this every plant of the new generation will contain the same complement of these genes but what happens to the genes if these di hybrids are self- fertilized if the genes are segregated independently into the sex cells and combined by chance a variety of combinations is possible there is a way to determine the probable combinations mathematically we know that in a simple Cross of tall and dwarf plants 3/4s will be tall and one quarter will be dwarf in a simple cross involving red and white flowers 3/4 will be red and one quar will be white mathematically the chances of tallness and redness occurring together can be expressed as the product of their chances of occurring separately thus the chance of a tall red combination is about 916 the chance of a tall white combination is 316 dwarf red 316 and dwarf white 1116 in an actual di hybrid cross the progeny do occur in the predicted ratios such a controlled cross and careful observation of progeny is called an inheritance test it was used by Gregor mle in his pioneering study of inheritance over a century ago it is now the basic tool of genetics if mandle had used 4:00 plants instead of Garden peas for an inheritance test he might have had different results clearly these hybrids are pink an intermediate color between the red and the white of the parent plants does this mean that inherited characteristics are sometimes Blended what happens to the genes if these hybrids are self-fertilized if each pair of genes in this plant consists of one white and one red Gene we would respect these Gene combinations one quar of the gene pairs would have red genes one quar would have red white combinations one quarter would have white red and one quarter would have white white an actual hybrid cross shows that one quarter of the plants in the new generation bear red flowers two quarters bear pink flowers and one quarter bear white the appearance of both red and white flowers demonstrates that genes are not Blended they continue to be transmitted in their original pure State through the sex cells of the parents the pink color must result from a pair of alals that are intermediate in expression rather than dominant and recessive The Inheritance test thus confirms our original premises the units of heredity genes exist in alternative forms called alals these genes occur in pairs genes segregate independently during the formation of sex cells and are recombined in the zygote through chance a gene pair may have one Alo that is dominant in expression but either a can be transmitted from generation to generation without being changed by the other when we understand these mechanisms of heredity we can predict statistically the occurrence of inherited characteristics of course the more differing hereditary characteristics involved in a given cross the more combinations are possible when only 10 pairs of genes are involved for example the number of possible combinations is in the hundreds of thousands every human being like many other organisms represents a combination of tens of thousands of gen gen pairs one gene of each pair being inherited from each parent that is how like begets like that is also how in the different species that inhabit the Earth infinite variety is possible