A2. Genetic Diagrams Simple Monohybrid Crosses Flashcards
What are genetic diagrams?
Diploid organisms have ___alleles for each gene. __________(sex cells) contain only one _________for each gene-they’re ________. When _________gametes from two parents _____together, the alleles they contain form the ___________of the ________ __________that is produced.
Genetic diagrams can be used to predict the _________and _______________of the __________ produced if two parents are crossed (bred). You need to know how to use genetic diagrams to interpret or predict the results of various crosses, including ______________ crosses.
Diploid organisms have two alleles for each gene. Gametes (sex cells) contain only one allele for each gene-they’re haploid. When haploid gametes from two parents fuse together, the alleles they contain form the genotype of the diploid offspring that is produced.
Genetic diagrams can be used to predict the genotypes and phenotypes of the offspring produced if two parents are crossed (bred). You need to know how to use genetic diagrams to interpret or predict the results of various crosses, including monohybrid crosses.
Monohybrid inheritance
Monohybrid inheritance is the inheritance of a ____________controlled by a ________ gene. Monohybrid ________show the ________of the __________ ________of that gene (and so different versions of the characteristic) being ___________by ___________of certain _____. The example below shows how wing length can be inherited in fruit flies.
Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. Monohybrid crosses show the likelihood of the different alleles of that gene (and so different versions of the characteristic) being inherited by offspring of certain parents. The example below shows how wing length can be inherited in fruit flies.
Monohybrid inheritance - Example pt1
The allele for normal wings is dominant, so it’s shown by a capital letter N. Any flies that have even one N allele will have normal wings. The allele for vestigial (little) wings is recessive, so it’s shown by the letter n. Only flies that have two n alleles will have vestigial wings.
The genetic diagram in Figure 2 shows a cross between one homozygous parent with normal wings (NN) and one homozygous parent with vestigial wings (nn). The normal winged parent can only produce gametes with the allele for normal wings (N). The vestigial winged parent can only produce gametes with the allele for vestigial wings (n).
Here’s how to draw a genetic diagram for this cross:…4 steps
Step 1: Make sure you’re clear what the letters mean.
Step 2: Show the parents’ genotypes at the top.
Step 3: The middle circles show the possible gametes. Put one of each letter into a circle.
Step 4: The lines show all the possible ways the gametes could combine. Fill in the possible combinations in the bottom boxes.
All offspring produced are heterozygous (Nn), as one allele is inherited from each parent.
Monohybrid inheritance - Example pt2
The genetic diagram in Figure 3 shows a cross between two parents from the F, generation (both heterozygous). Just follow the same steps as on the previous page, but this time the gametes produced by each F, offspring may contain the allele for either normal (N) or vestigial wings (n).
Figure 3: Genetic diagram of a monohybrid cross between heterozygous parents.
Phenotypic ratios
The phenotypic ratio is the ratio of different phenotypes in the offspring. Genetic diagrams allow you to predict the phenotypic ratios in __ and
__ offspring.
Usually whenever you do a monohybrid cross with two heterozygous parents you get a : ratio of dominant: recessive characteristics in the offspring. However, sometimes you won’t get the expected (predicted) phenotypic ratio. For example, codominant alleles and sex linkage can both alter phenotypic ratios in the offspring of monohybrid crosses.
The phenotypic ratio is the ratio of different phenotypes in the offspring. Genetic diagrams allow you to predict the phenotypic ratios in F1 and
F2 offspring.
Usually whenever you do a monohybrid cross with two heterozygous parents you get a 3:1 ratio of dominant: recessive characteristics in the offspring. However, sometimes you won’t get the expected (predicted) phenotypic ratio. For example, codominant alleles and sex linkage can both alter phenotypic ratios in the offspring of monohybrid crosses.
Punnett squares - example (3 steps)
Step 1: Work out the alleles the gametes would have.
Step 2: Cross the parents’ gametes to show the possible genotypes of the F, generation -all heterozygous, Nn.
Step 3: Cross the gametes of the F, generation to show the possible genotypes of the F, generation. The Punnett square shows a 75% chance that offspring will have normal wings and a 25% chance that they’ll have vestigial wings, i.e. a 3:1 ratio.
Monohybrid inheritance of codominant alleles
Occasionally, alleles show codominance -_____ _________are ____________in the _____________, and _________ one is ___________. One example in humans is the ________for sickle-cell _________, a genetic disorder caused by a __________ in the _______________ gene. It causes red blood cells to be sickle-shaped.
Occasionally, alleles show codominance -both alleles are expressed in the phenotype, and neither one is recessive. One example in humans is the allele for sickle-cell anaemia, a genetic disorder caused by a mutation in the haemoglobin gene. It causes red blood cells to be sickle-shaped.
Monohybrid inheritance of codominant alleles - example
People who are homozygous for normal haemoglobin (HHN) don’t have the disease. People who are homozygous for sickle haemoglobin (HH) have sickle-cell anaemia-all their blood cells are sickle shaped. People who are heterozygous (HNHS) have an in-between phenotype, called the sickle-cell trait-they have some normal haemoglobin and some sickle haemoglobin. The two alleles are codominant because they’re both expressed in the phenotype. The genetic diagram in Figure 5 shows the possible offspring from crossing two parents with sickle-cell trait (heterozygous).
This cross has produced a ….
This cross has produced a 1:2:1 phenotypic ratio of 1 unaffected: 2 sickle-cell trait: 1 sickle-cell anaemia, or 1 unaffected homozygous: 2 heterozygous : 1 disorder homozygous.
Tip: When alleles show codominance they’re represented in a slightly different way to normal -you show the main gene as a normal capital letter and then the alleles as superscript capitals, because neither is ____________.
Tip: When alleles show codominance they’re represented in a slightly different way to normal -you show the main gene as a normal capital letter and then the alleles as superscript capitals, because neither is recessive.
Exam Tip If you’re not given letters
to use for a genetic diagram, just choose sensible ones yourself, eg. T for tall dominant allele and t for dwarf recessive allele. Try to avoid using letters that look similar as capital and lower case letters, e.g. Cand c.
Exam Tip If you’re not given letters
to use for a genetic diagram, just choose sensible ones yourself, eg. T for tall dominant allele and t for dwarf recessive allele. Try to avoid using letters that look similar as capital and lower case letters, e.g. Cand c.
