Term 1 review: reactions, electricity and heredity
الأسبوع 15 · مراجعة الفصل الدراسي الأول (الوحدات ١–٤) ومهام مراجعة من إعداد Darslab · 3 دروس
3
Review: heredity, selection, fossils and rock dating
Science Prep 3 Term 1, Units 1–4: Review
GoalReview heredity, selection, fossils and rock dating with new mixed tasks. No new facts.
New words
Punnett square · a box diagram that shows the possible offspring of a crossnatural selection · traits that fit the environment become more common over generationshalf-life · the time for half of the radioactive atoms to change
Explanation
Remember: in a Punnett square, Rr × Rr gives 25 % RR, 50 % Rr, 25 % rr; a dihybrid cross gives 9 : 3 : 3 : 1 in 16 boxes. Carriers are healthy, and inherited diseases are not catching. Populations grow like an S until they reach the carrying capacity. Natural selection works on variation in a population over generations; artificial selection is the choice of people. Fossils are clues in sedimentary rock; the lowest layer is the oldest. The geologic time scale and the half-life give good estimates of age, not exact ones. Talk about evidence politely.
P: ♀ Tt × ♂ tt
♀ \ ♂
t
T
Tt
t
tt
Genotype: 1 Tt : 1 tt Phenotype: 1 : 1
Worked examples
Tt × tt: the offspring are Tt and tt in equal numbers: 50 % tall and 50 % short if T is tall. Of 60 seeds, about 30 are tall.
A population: 5 flies double 3 times: 5 × 2³ = 40. In a real jar the carrying capacity would stop the growth.
Dating: 200 atoms, half-life 4 000 years. After 8 000 years there are 200 ÷ 2² = 50 left. An age found from 50 left would be about 8 000 years, a good estimate.
Class activity
Final review page. Make a one-page map with three boxes (heredity, selection and populations, fossils and dating). In each box put one drawing or formula and one new question. Choose a task from each box and do it. Then write which lesson of this term you want to practise more and why.
Practice
1
Rr × Rr: what fraction of offspring are rr?
1/4.
2
From RrYy × RrYy, how many of 160 offspring are expected to be round and yellow?
9/16 × 160 = 90.
3
Which selection is done by people?
Artificial selection.
4
Which layer in an undisturbed cliff is the oldest?
The bottom one.
5
120 atoms with half-life 3 000 years. How many remain after 6 000 years?
