Mendelian Patterns of Inheritance. Chapter 11. Mendel's Law of Segregation (MONOHYBRID CROSS). A monohybrid cross involves one (mono) character and different (hybrid) traits. The F1 seeds were all purple; the white flower trait failed to appear at all.
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Mendelian Patterns of Inheritance
Because the purple flower trait completely masks the white flower trait when true-breeding plants are crossed, the purple flower trait is called dominant, and the white flower trait is called recessive.
Creating the F2 generation
*Cross the F1 generation together to create F2
*Ratio is always 3:1
Mendel proposed that the units responsible for inheritance were discrete particles - particulate theory of inheritance
The same pattern occurred over and over, for seven different traits.
PP x pp
purple x white
The F1 cross
Pp x Pp
purple x purple
When an organism is studied for three different genes and has the alleles AABbCC, it is homozygous for A and C genes but heterozygous for the B gene.
A = antennaa = no antenna
B = black
b = purple
C = chompersc = no chompers
The physical appearance of an organism is its phenotype. Purple-flowered would be a phenotype.
The actual composition of the organism's alleles for a gene is its genotype: Pp is a genotype.
Organisms have many different genes some have thousands, and complex organisms have 10 times that number.
The dominant trait is given a capitol letter, the lowercase of that same letter is the recessive trait. DO NOT MIX LETTERS. Pick one and stick to it.
Also, some letters are better than others. Capital S looks a lot like a lowercase (s). Pick a different letter...
Okay Better (use H for hair)
Short hair = SS HH
Short hair = Ss Hh
Long hair = ss hh
1. Polka dots are dominant to stripes.
2. Long sleeves are dominant to short sleeves.
3. Collared shirts are recessive.
1. A round seeded plant (RR) is crossed with a wrinkle seeded plant (rr). What are the phenotypes of the offspring?
2. Two heterozygous purple flowered pea plants are crossed. What are the phenotypes of their offspring and in what proportion?
3. A plant with green seeds (yy) is crossed with a heterozygous plant. What percentage of their offspring have yellow seeds?
Wings are a dominant trait, but some dragons are born wingless.
What are the chances that two heterozygous dragons have a whelp that is wingless?
If a wingless dragon is crossed with one that is heterozygous, how many of its offspring will also be wingless?
I can help you! Let's have offspring!
Help, help! I don't know what my genotype is!!
Am I Dd or DD?
Why does the punnett square work?
It all goes back to meiosis.. each side represents a sperm or egg. The boxes filled out simply give you the statistical chance that a certain sperm will fertilize a certain egg.
Consider a pea plant that is RrTt (round seed, tall)
When this plant's cells go through MEIOSIS, the alleles segregate - each sperm receives a random combination...
R T R t
r T r t
Mendel’s Three Laws 1. Dominance & Recessiveness 2. Segregation
3. Independent Assortment
The second law describes the outcome of dihybrid (two character) crosses, or hybrid crosses involving additional characters.
A dihybrid is an individual that is a double heterozygote (e.g., with the genotype RrYy - round seed, yellow seed).
What are the gametes that can be produced by this individual?
AaBb x AaBb
9 - (two dominant traits)
3 - (one dominant, one recessive)
3 - (one recessive, one dominant)
1 - (two recessive traits)
A Mathematical Alternative (LAWS OF PROBABILITY)
A punnet square is not needed to determine the ratios of genotypes and phenotypes. Simple statistics and math can save you the trouble of filling out a square.
In a monohybrid cross Pp x Pp, each parent produced P gametes and p gametes
If you wanted to determine how many of the offspring are pp: x =
H is dominate for long hair (h = short) and B is dominate for black eyes (b = red eyes). If the parents are...
HhBb x hhBb
How many off the offspring will be short haired and red eyed?
Winged, Fire breathing dragon DdFf
Wingless, Fire breathing dragon ddFf