Chapter 17- Inherited Change Flashcards
3.7.1 (72 cards)
What is the significance of Mendel in inheritence?
Gregor Mendel- work rediscovered to show the laws characteristics are inherited by.
What is the genotype?
- Genetic constitution of an organism- all the alleles an organism has.
- Determines the limits within which the characteristics of an individual may vary, however, the actual characteristics of the individual are affected by other elements, such as diet.
- E.g. lack of calcium leading to maximum height not being reached.
What is the phenotype?
- Observable/ biochemical characteristics of an organism.
- The expression due to the genetic constitution and its interaction with the environment.
- Expression of the genotype and the environmental effects can alter the phenotype.
What is a gene?
- Length of DNA- sequence of nucleotide bases on a DNA molecule that code for a particular polypeptide (or RNA).
- Polypeptides may then determine characteristics e.g., enzymes, pigments. All individuals of the same species have the same genes.
Describe the features of alleles.
- Individuals of a species share the same genes but (usually) different combinations of alleles of these genes.
- An individual inherits alleles from their parent or parents.
- Allele- one of the different forms of a gene. Most plants and animals carry two alleles of each gene- one from each parent- may be more.
- There may be many alleles of a single gene.
- Order of bases in each allele slightly different- each allele codes for different versions of the same characteristic.
- Represented using letters.
What is a locus?
- Fixed position of a gene on a DNA molecule.
- Only one allele of a gene can occur at the locus of any one chromosome.
- Diploid organisms- two copies of each chromosome- alleles of each gene are found at the same locus on each chromosome in a pair.
What are homologous chromosomes?
Chromosomes occur in pairs in diploid organisms- two loci that carry one allele of a gene- two alleles in total- one from mother and one from father.
Name and describe the two types of allele combinations.
- Homozygous- allele on each chromosome of the diploid organism is the same- carries two copies of the same allele at the specific locus.
- Heterozygous- allele on each chromosome of the diploid organism is different- carries two different alleles for a gene at a specific locus. Usually only one gene shows itself in the phenotype.
Name and describe the two idfferent types of alleles.
- Dominant- the allele of the heterozygote that is always expressed in the phenotype even when there’s only one copy- shown by capital letters.
- Recessive- the allele in a heterozygote that is not expressed- only appears in the phenotype if two copies present- shown by lower case letters.
Name and describe the possible ararngements of alleles.
- Homozygous dominant- homozygous organism with two dominant alleles.
- Homozygous recessive- homozygous organism with two recessive alleles. The only time the recessive allele is expressed in the diploid organism.
- Codominant-** two alleles both expressed in the phenotype** as neither one is recessive. When both alleles occur the alleles either blend together (e.g. roan coat from red and white hairs) or both features are represented (e.g. both A and B antigens in blood group AB).
Describe the meaning of multiple alleles.
- Gene with more than two allele forms.
- As there are always only two chromosomes in a homologous pair, only two out of the three or more alleles can be present in one organism. E.g. ABO blood group.
Draw a diagram showing how the terms for inheritance interlink.
Answer on revision card.
Describe how fertilisation works with inheritance.
- Two alleles for each gene.
Gametes only contain one allele for each gene- haploid. When haploid gametes fuse together- alleles they contain form the genotype of diploid offspring produced.
What are genetic diagrams?
Diagrams that predict genotypes and phenotypes of offspring when two parents are crossed.
What is monohybrid inheritence?
Inheritance of characteristic controlled by a single gene e.g. colour of pea pods.
What are monohybrid crosses?
Show the likelihood of different alleles of a gene (and so different versions of the characteristic) being inherited by offspring of certain parents in monohybrid inheritance. Crosses may be used instead of punnet squares so know how to use both.
How should you perform a monhybrid cross?
Hint: 7 steps
- The question usually gives the symbols- use a single letter to represent each characteristic. If doesn’t give you symbol use the first letter of each one of the different features, but try to choose one where the upper and lower case are distinguishable.
- Upper case- represents dominant feature. Lower case- represents recessive feature. Never use two different letters where one character is dominant. The upper case always goes first.
- Make it clear what the letters mean first. Use appropriate letters to make dominant/ recessive alleles clear- avoid s, w, u, o, p, z, x, c, v.
- Represent parents’ genotypes and phenotypes with appropriate letters- label them as parents.
- State the gametes- label them as gametes and circle them.
- Use a Punnett square to show the results of the random crossing. Label male and female gametes, keeping them encircled. May also need to use genetic cross- write out all the possible combinations in the bottom boxes.
- State the phenotype of each genotype and indicate the number of each type.
What does pure breeding mean?
If organisms with the same characteristic are bred repeatedly with another so that they consistently produce the same feature. They are homozygous for the allele that produces the characteristic.
Describe what F1 means and how it results in the monohybrid cross ratio?
- If a pure breeding dominant is bred with a pure breeding recessive the offspring are known as the first filial or F1 generation. All the offspring will express the dominant allele as they will all be heterozygous.
- When the heterozygous organisms from the F1 generation are crossed the offspring known as the second filial- F2- generation are always in the ratio 3:1 for monohybrid inheritence where the 3 is dominant and the 1 is recessive.
- The larger the number of offspring the more likely the ratio will reach 3:1.
What is the basic law of genetics?
In diploid organisms characteristics are determined by alleles that occur in pairs. Only one of each pair of alleles can be present in a single gamete.
What are phenotype ratios?
Ratio of different phenotypes in the offspring. Genetic diagrams allow you to predict the phenotypic ratios in F1 and F2 generations
What is the phenotypic ratio for a genetic cross.
Two heterozygous parents in monohybrid cross- F2 generation- 3:1 ratio of dominant : recessive characteristics.
When may the expected phenotypic ratio not occur for monohybrid crosses?
In codominant alleles and sex linkage- alter phenotypic ratios in monohybrid crosses.
Perform a monohybrid cross for a homozygous recessive and homozygous dominant pea plant. Green is dominant and yellow is recessive.
Answer on revision card.