3.4 INHERITANCE
Nature of science:
Making quantitative measurements with replicates to ensure reliability. Mendel’s genetic crosses with pea plants generated numerical data. (3.2)
Understandings:
• Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.
• Gametes are haploid so contain only one allele of each gene.
• The two alleles of each gene separate into different haploid daughter nuclei during meiosis.
• Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.
• Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.
• Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles.
• Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes.
• Many genetic diseases have been identified in humans but most are very rare.
• Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.
• Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.
• Gametes are haploid so contain only one allele of each gene.
• The two alleles of each gene separate into different haploid daughter nuclei during meiosis.
• Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.
• Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.
• Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles.
• Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes.
• Many genetic diseases have been identified in humans but most are very rare.
• Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.
key terminology
gregor mendel
gametes have only one allele of each gene
A punnet grid can be used to show how the alleles of parents are split between their gametes and how combinations of alleles can show up.
5 steps of the punnet grid method
1. choose a letter
2. Parent's genotypes (TT or tt)
3. Determine the gametes that the parents can produce
4. Draw the punnett grid
5. Work out the chances of each genotype and phenotype occurring
No matter what the outcome, each offspring is the result of 2 alleles coming together when the gametes fuse. In this process, the 2 haploid sex cells join to make a single diploid cell called zygote. This is the first cell of the new offspring.
2. Parent's genotypes (TT or tt)
3. Determine the gametes that the parents can produce
4. Draw the punnett grid
5. Work out the chances of each genotype and phenotype occurring
No matter what the outcome, each offspring is the result of 2 alleles coming together when the gametes fuse. In this process, the 2 haploid sex cells join to make a single diploid cell called zygote. This is the first cell of the new offspring.
test cross
A plant breeder might need to know whether a specific plant is a purebred for tallness (TT) of whether it will not breed true for tallness (Tt). To find out, he will cross the tall plant with a plant whose genotype is definitely known ( a short plant that must be homozygous recesive tt). By looking at the resulting plants the test cross can reveal the genotype of the tall plant is either TT or Tt
multiple alleles
So far only 2 possibilities have been considered for a gene: dominant or recessive. With 2 alleles, 3 different genotypes are possible, which can produce 2 different phenotypes. However sometimes there are 3 or more alleles fro the same gene. This is the case for alleles that determine the blood type
Blood types
•Red blood cells: transport oxygen
•White blood cells: help fight infection
•Platelets: help blood clot
•Plasma: medium that carries these cells
Each blood type has an specific:
•Antigen = surface markers on your red blood cell (your blood cell’s ID)
•Antibody = proteins produced by white blood cells, recognize certain antigens and trigger an immune response
There are 4 possible phenotypes: A, B, AB or O. To create these 4 blood types there are 3 alleels of the gene. These 3 alleles can produce 6 different genotypes.
•White blood cells: help fight infection
•Platelets: help blood clot
•Plasma: medium that carries these cells
Each blood type has an specific:
•Antigen = surface markers on your red blood cell (your blood cell’s ID)
•Antibody = proteins produced by white blood cells, recognize certain antigens and trigger an immune response
There are 4 possible phenotypes: A, B, AB or O. To create these 4 blood types there are 3 alleels of the gene. These 3 alleles can produce 6 different genotypes.
Autosomal genetic disease in humans
Autosomal recessive diseases are the ones cause by recessive alleles, and the locus of their genes is found on one of the first 22 pairs of chromosomes but not on the sex chromosomes X or Y. The examples of autosomal recessive diseases are: albinism, cystic fibrosis, phenykeltonuria, sickle cell disease and sickle cell trait, tay Sachs disease, and thalassemia. This genetic diseases are very rare, the most frequently occurring autosomal recessive diseases only affect 1 in 2000 people.
diseases caused by sex-linked genes or co-dominant allels
Genes carried on the sex chromosome: Because the Y chromosome is significantly smaller than the X chromosome, it has fewer loci and therefore fewer genes than the X chromosome. This means that sometimes alleles present on the X chromosome have nothing to pair up with.
Sex Linkage: Any genetic trait whose gene has its locus on the X or Y chromosome is said to be sex linked. Often genetics traits that show sex linkage affect one sex more than the other. Examples of this genetic traits are colour blindness and hemophilia.
-Color blindness: is the inability to distinguish between certain colors.
-Hemophilia: is a disorder in which blood doesn’t clot properly.
Sex Linkage: Any genetic trait whose gene has its locus on the X or Y chromosome is said to be sex linked. Often genetics traits that show sex linkage affect one sex more than the other. Examples of this genetic traits are colour blindness and hemophilia.
-Color blindness: is the inability to distinguish between certain colors.
-Hemophilia: is a disorder in which blood doesn’t clot properly.
The pattern of inheritance with sex-linked genes
Carriers of sex-linked traits
Sex-linked recessive alleles such as X are rare in most populations of human worldwide. It is very unlikely to get one andmuch less likely to get 2 such alleles. This is why few women are color blind given that their second copy of the gene is likely to be the dominant allele for full color vision and will mask the recessive allele. The same is true for hemophilia.
There are 3 possible genotypes for females but only 2 possible genotypes for males. Only women can be heterozygous and as a result they are the only ones who can be carriers.
Because men do not have a second X chromosome there are only 2 possible genotypes (the x dominant or recessive) for them in relation to color blindness. With just the one recessive allele a man will be color blind. This is contrary to what you have seen up to now concerning recessive alleles, usually people need two to have the trait, and with one they are carriers. In this case, the single recessive allele in males determines the phenotype. Men cannot be carriers for X-linked alleles.
Sex-linked recessive alleles such as X are rare in most populations of human worldwide. It is very unlikely to get one andmuch less likely to get 2 such alleles. This is why few women are color blind given that their second copy of the gene is likely to be the dominant allele for full color vision and will mask the recessive allele. The same is true for hemophilia.
There are 3 possible genotypes for females but only 2 possible genotypes for males. Only women can be heterozygous and as a result they are the only ones who can be carriers.
Because men do not have a second X chromosome there are only 2 possible genotypes (the x dominant or recessive) for them in relation to color blindness. With just the one recessive allele a man will be color blind. This is contrary to what you have seen up to now concerning recessive alleles, usually people need two to have the trait, and with one they are carriers. In this case, the single recessive allele in males determines the phenotype. Men cannot be carriers for X-linked alleles.
Theory of knowledge:
• Mendel’s theories were not accepted by the scientific community for a long time. What factors would encourage the acceptance of new ideas by the scientific community?
Applications and skills:
• Application: Inheritance of ABO blood groups.
• Application: Red-green colour blindness and hemophilia as examples of sex- linked inheritance.
• Application: Inheritance of cystic fibrosis and Huntington’s disease.
• Application: Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.
• Skill: Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
• Skill: Comparison of predicted and actual outcomes of genetic crosses using real data.
• Skill: Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases.
• Mendel’s theories were not accepted by the scientific community for a long time. What factors would encourage the acceptance of new ideas by the scientific community?
Applications and skills:
• Application: Inheritance of ABO blood groups.
• Application: Red-green colour blindness and hemophilia as examples of sex- linked inheritance.
• Application: Inheritance of cystic fibrosis and Huntington’s disease.
• Application: Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.
• Skill: Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
• Skill: Comparison of predicted and actual outcomes of genetic crosses using real data.
• Skill: Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases.